I'm making a simple guestbook script in php. I pretty much have it finished but need help finding the logic errors in my code. One is that when echoing the results from the form content. I am wanting to display the email, whole name and comment, each on different lines.
like this
email: some#email.com
Name: joe somebody
Comment:
hello world.
but for some reason it just displays the first and last name on all lines.
the other 3 issues are sorting(ascending, descending) and removing duplicates.
Thanks in advance for your advice on how to fix this.
I just ask you be detailed so i know what you are talking about.
In any case, heres the code:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Guest Book</title>
</head>
<body>
<?php
// is there data in the fields
if (isset($_POST['submit'])){
$firstName = addslashes($_POST['first_name']);
$lastName = addslashes($_POST['last_name']);
$wholeName = $firstName . "," . $lastName;
$email = addslashes($_POST['email']);
$comment = addslashes($_POST['comment']);
$guestRecord = "$wholeName~$email~$comment\n";
$guestBook = fopen("guestbook.txt", "ab");
//if not let the user know
if ($guestBook === FALSE)
echo "There was an error saving your entry!\n";
else {
// if there is info add it to the txt file
fwrite($guestBook, $guestRecord);
// close the txt file
fclose($guestBook);
//let the user know info was saved
echo "Your entry has been saved.\n";
}
}
?>
<h2>Enter your name to sign our guest book</h2>
<hr>
<form method="POST" action ="GuestBook1.php">
<p>First Name<input type ="text" name="first_name"/></p>
<p>Last Name<input type ="text" name="last_name"/></p>
<p>Email<input type ="text" name="email"/></p>
<p><textarea name="comment" id="comment"/></textarea></p>
<p><input type ="submit" value ="Submit" name="submit"/></p>
</form>
<hr>
<p>Show Guest Book
<br />
Delete Guest Entry
<br />
Remove Duplicate Entries
<br />
Sort Entries A-Z
<br />
Sort Entries Z-A
<br />
</p>
<?php
// if theres info in the form process info
if (isset($_GET['submit'])) {
if ((file_exists("guestbook.txt")) &&
(filesize("guestbook.txt") != 0)) {
$guestArray = file("guestbook.txt");
//switch to $_Get Method
switch ($_GET['submit']) {
// remove duplicate entries
case 'Remove Duplicates':
$guestArray = array_unique($guestRecord);
$guestArray = array_values($guestRecord);
break;
//sort ascending
case 'Sort Ascending':
$guestArray = sort($guestArray);
break;
//sort Decending
case 'Sort Decending':
$guestArray = ksort($guestArray);
break;
}
//count to see how many entries there are
if (count($guestArray)>0) {
$newGuest = implode($guestArray);
$guestStore = fopen("guestbook.txt", "ab");
if ($guestStore === false)
echo "There was an error updating the message file\n";
else {fwrite($guestStore, $newGuest);
fclose($guestStore);
}
}
else
unlink("guestbook.txt");}
}
if ((!file_exists("guestbook.txt")) ||
(filesize("guestbook.txt") == 0))
echo "<p>There are no entries.</p>\n";
else {
// there isnt anything in the txt file show an error
if ((!file_exists("guestbook.txt")) ||
(filesize("guestbook.txt") == 0))
echo "<p>There are no entries.</p>\n"; else {
//if there is information display the txt file in a table
$guestArray = file("guestbook.txt");
echo "<table style=\"background-color:lightgray\"
border=\"1\" width=\"100%\">\n";
//begin counting number of guest entries
$count = count($guestArray);
for ($i = 0; $i < $count; ++$i) {
$currMsg = explode("~", $guestArray[$i]);
// display results in a table
echo "<td width=\"5%\" style=\"text-align:center;
font-weight:bold\">" . ($i + 1) . "</td>\n";
echo "<td width=\"95%\"><span style=\"font-weight:bold\">
Email:</span> " . htmlentities($currMsg[0]) . "<br />\n";
echo "<span style=\"font-weight:bold\">Name:</span> " .
htmlentities($currMsg[0]) . "<br />\n";
echo "<span style=\"text-decoration:underline;
font-weight:bold\">Comment</span><br />\n" .
htmlentities($currMsg[0]) .
"</td>\n";
echo "</tr>\n";
}
echo "</table>\n";
}
}
?>
</body>
</html>
$firstName = addslashes($_POST['first_name'])."\r\n";
The \r\n\ will put things physically on a new line. Make sure to use double quotes.
Regarding your first issue, you are echoing $currMsg[0] for all of the fields. You need to use the correct index for each of the values (0 for name, 1 for email, 2 for comment). Or even better, use something like:
list($name, $email, $comment) = explode("~", $guestArray[$i]);
That way you have meaningful variable names.
The other issues you'll need to be more specific as to what you're trying to do (and in what way your current code isn't working).
I can see at least one issue though - your links have ?action= but your switch statement is looking at $_GET['submit'], not $_GET['action']. Plus you have a typo in case 'Sort Decending': (missing 's')
Related
Can someone help me to figure out how to replace a defined string value with user input value? I am quite new in PHP programming and could not find an answer. I saw a lot of ways to replace string on the internet by using built-in functions or in arrays, but I could not find out the right answer to my question.
Here is my code:
$text = "Not found";
if ( isset($_GET['user'])) {
$user_input = $_GET['user'];
}
// from here I I tried to replace the value $text to user input, but it does not work.
$raw = TRUE;
$spec_char = "";
if ($raw) {
$raw = htmlentities($text);
echo "<p style='font-style:bold;'> PIN " . $raw . "</p>"; *# displays "Not found"*
} elseif (!$raw == TRUE ) {
$spec_char = htmlspecialchars($user_input);
echo "<p>PIN $spec_char </p>";
}
<form>
<input type="text" name="user" size="40" />
<input type="submit" value="User_val"/>
</form>
I appreciate your answers.
Lets run over your code, line by line.
// Set a default value for $text
$text = "Not found";
// Check if a value has been set...
if (isset($_GET['user'])) {
// But then create a new var with that value.
// Why? Are you going to change it?
$user_input = $_GET['user'];
}
// Define a few vars
$raw = TRUE;
$spec_char = "";
// This next line is useless - Why? Because $raw is always true.
// A better test would be to check for $user_input or do the
// isset() check here instead.
if ($raw) {
// Basic sanity check, but $text is always going to be
// "Not found" - as you have never changed it.
$raw = htmlentities($text);
// render some HTML - but as you said, always going to display
// "Not found"
echo "<p style='font-style:bold;'> PIN " . $raw . "</p>";
} elseif (!$raw == TRUE ) {
// This code is never reached.
$spec_char = htmlspecialchars($user_input);
echo "<p>PIN $spec_char </p>";
}
// I have no idea what this HTML is for really.
// Guessing this is your "input" values.
<form>
<input type="text" name="user" size="40" />
<input type="submit" value="User_val"/>
</form>
Just a guess I think you really wanted to do something more like this:
<?php
// Check if a value has been posted...
if (isset($_POST['user'])) {
// render some HTML
echo '<p style="font-style:bold"> PIN '.htmlspecialchars($_POST['user']).'</p>';
}
?>
<form method="post" action="?">
<input type="text" name="user" size="40" />
<input type="submit" value="User_val"/>
</form>
I have a form which the user inputs a file name into. It iterates through all the directories successfully looking to match the users search input to the relevant pdf file.
When it finds the match it correctly echos 'it matches' and breaks out of the foreach loop. However, it also correctly states 'not a match' for all the files it finds before it matches the correct file. So I get a long list of 'not a match ' followed by 'it matches'.
If I echo " " instead of 'not a match' it works fine and doesn't display anything but I would like to tell the user only once that what they have inputted does not match. I am sure I have overlooked something basic but any assistance would be greatly appreciated on how to achieve this.
Here is the code I have.
<?php
if (isset($_POST['submit']) && !empty($_POST['target'])) {
$searchInput = $_POST['target'];
$it = new RecursiveDirectoryIterator("/myDirectory/");
foreach(new RecursiveIteratorIterator($it) as $file) {
$path_info = pathinfo($file);
$extension = $path_info['extension'];
if (strcmp("pdf", $extension) == 0) {
$lowerInput = strtolower($searchInput);
if (!empty($path_info)) {
$string = strtolower($path_info['filename']);
if(preg_match("~\b" . $lowerInput. "\b~", $string)) {
echo "it matches <br>";
break;
} else {
if (!preg_match("~\b" . $lowerInput . "\b~", $string)) {
echo "not a match <br>";
}
}
}
}
} //end foreach
} // end if submit pressed
?>
<html>
<head>
</head>
<body>
<h3>Search Files</h3>
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>" id="searchform">
Type the File You Require:<br><br>
<input id="target" type="text" name="target" required >
<br>
<input id="submit" type="submit" name="submit" value="Search" >
</form>
</body>
</html>
I've left out some of your code, just showing the parts that matter. Basically just set a variable and echo it once when the foreach completes.
$msg="Not a match <br>" ;
foreach(new RecursiveIteratorIterator($it) as $file) {
...
if(preg_match("~\b" . $lowerInput. "\b~", $string)) {
$msg = "it matches <br>" ;
break ;
}
// no need for an else clause, "not a match" is default finding
}// end of foreach
echo $msg ;
My code should provide two random numbers and have the user enter their product (multiplication).
If you enter a wrong answer, it tells you to guess again, but keeps the same random numbers until you answer correctly. Answer correctly, and it starts over with a new pair of numbers.
The below code changes the value of the two random numbers even if I entered the wrong number. I would like to keep the values the same until the correct answer is entered.
<?php
$num1=rand(1, 9);
$num2=rand(1, 9);
$num3=$num1*$num2;
$num_to_guess = $num3;
echo $num1."x".$num2."= <br>";
if ($_POST['guess'] == $num_to_guess)
{ // matches!
$message = "Well done!";
}
elseif ($_POST['guess'] > $num_to_guess)
{
$message = $_POST['guess']." is too big! Try a smaller number.";
}
elseif ($_POST['guess'] < $num_to_guess)
{
$message = $_POST['guess']." is too small! Try a larger number.";
}
else
{ // some other condition
$message = "I am terribly confused.";
}
?>
<!DOCTYPE html>
<html>
<body>
<h2><?php echo $message; ?></h2>
<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="hidden" name="answer" value="<?php echo $answer;?>">
<input type="hidden" name="expression" value="<?php echo $expression;?>">
What is the value of the following multiplication expression: <br><br>
<?php echo $expression; ?> <input type="text" name="guess"><br>
<input type="submit" value="Check">
</form>
</body>
</html>
In order to keep the same numbers, you have to store them on the page and then check them when the form is submitted using php. You must also set the random number if the form was never submitted. In your case, you were always changing num1 and num2. I tried to leave as much of your original code intact, but it still needs some work to simplify it.
First I added 2 more hidden field in the html called num1 and num2
Second, I set $num1 and $num2 to the value that was submitted from the form.
After following the rest of the logic, I make sure that $num1 and $num2 are reset if the answer is correct of it the form was never submitted.
You can see the comments in the code below.
Additionally, if you were going to use this in a production environment, you would want to validate the values being passed in from the form so that malicious users don't take advantage of your code. :)
<?php
// Setting $num1 and $num2 to what was posted previously and performing the math on it.
$num1 = $_POST['num1'];
$num2 = $_POST['num2'];
$num_to_guess = $num1*$num2;
// Check for the correct answer
if ($_POST && $_POST['guess'] == $num_to_guess)
{
// matches!
$message = "Well done!";
$num1=rand(1, 9);
$num2=rand(1, 9);
}
// Give the user a hint that the number is too big
elseif ($_POST['guess'] > $num_to_guess)
{
$message = $_POST['guess']." is too big! Try a smaller number.";
}
// Give the user a hint that the number is too small
elseif ($_POST['guess'] < $num_to_guess)
{
$message = $_POST['guess']." is too small! Try a larger number.";
}
// If the form wasn't submitted i.e. no POST or something else went wrong
else
{
// Only display this message if the form was submitted, but there were no expected values
if ($_POST)
{
// some other condition and only if something was posted
$message = "I am terribly confused.";
}
// set num1 and num2 if there wasn't anything posted
$num1=rand(1, 9);
$num2=rand(1, 9);
}
// Show the problem
echo $num1."x".$num2."= <br>";
?>
<!DOCTYPE html>
<html>
<body>
<h2><?php echo $message; ?></h2>
<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="hidden" name="num1" value="<?= $num1 ?>" />
<input type="hidden" name="num2" value="<?= $num2 ?>" />
<input type="hidden" name="answer" value="<?php echo $num3;?>">
<input type="hidden" name="expression" value="<?php echo $expression;?>">
What is the value of the following multiplication expression: <br><br>
<input type="text" name="guess"><br>
<input type="submit" value="Check">
</form>
</body>
</html>
When you load the page for the first time, you have (i.e.) “2*3” as question. $_POST is not defined, so if ($_POST['guess']... will produce a undefined index warning. Then you echo $message, but where you define $message? $_POST['guess'] is undefined, so is evaluated as 0, $num_to_guess is 6 (=2*3), so $message is set to " is too small! Try a larger number.", even if the user has not input anything. The hidden answer is set to $answer, but this variable is not defined so it is set to nothing (or to “Notice: Undefined variable: answer”, if you activate error reporting). Same issue for expression input and for echo $expression.
Try something like this:
$newQuestion = True; // This variable to check if a new multiplication is required
$message = '';
/* $_POST['guess'] check only if form is submitted: */
if( isset( $_POST['guess'] ) )
{
/* Comparison with answer, not with new result: */
if( $_POST['guess'] == $_POST['answer'] )
{
$message = "Well done!";
}
else
{
/* If result if wrong, no new question needed, so we propose same question: */
$newQuestion = False;
$answer = $_POST['answer'];
$expression = $_POST['expression'];
if( $_POST['guess'] > $_POST['answer'] )
{
$message = "{$_POST['guess']} is too big! Try a smaller number.";
}
else
{
$message = "{$_POST['guess']} is too small! Try a larger number.";
}
}
}
/* New question is generated only on first page load or if previous answer is ok: */
if( $newQuestion )
{
$num1 = rand( 1, 9 );
$num2 = rand( 1, 9 );
$answer = $num1*$num2;
$expression = "$num1 x $num2";
if( $message ) $message .= "<br>Try a new one:";
else $message = "Try:";
}
?>
<!DOCTYPE html>
(... Your HTML Here ...)
This might also be fun to learn. This is a session. Lets you store something temporarily. It is a little dirty. But fun to learn from.
http://www.w3schools.com/php/php_sessions.asp
<?php
session_start(); // Starts the Session.
function Save() { // Function to save $num1 and $num2 in a Session.
$_SESSION['num1'] = rand(1, 9);
$_SESSION['num2'] = rand(1, 9);
$_SESSION['num_to_guess'] = $_SESSION['num1']*$_SESSION['num2'];;
$Som = 'Guess the number: ' . $_SESSION['num1'] .'*' .$_SESSION['num2'];
}
// If there is no session set
if (!isset($_SESSION['num_to_guess'])) {
Save();
$message = "";
}
if (isset($_POST['guess'])) {
// Check for the correct answer
if ($_POST['guess'] == $_SESSION['num_to_guess']) {
$message = "Well done!";
session_destroy(); // Destroys the Session.
Save(); // Set new Sessions.
}
// Give the user a hint that the number is too big
elseif ($_POST['guess'] > $_SESSION['num_to_guess']) {
$message = $_POST['guess']." is too big! Try a smaller number.";
$Som = 'Guess the number: ' . $_SESSION['num1'] .'*' .$_SESSION['num2'];
}
// Give the user a hint that the number is too small
elseif ($_POST['guess'] < $_SESSION['num_to_guess']) {
$message = $_POST['guess']." is too small! Try a larger number.";
$Som = 'Guess the number: ' . $_SESSION['num1'] .'*' .$_SESSION['num2'];
}
// some other condition
else {
$message = "I am terribly confused.";
}
}
?>
<html>
<body>
<h2><?php echo $Som . '<br>'; ?>
<?php echo $message; ?></h2>
<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="text" name="guess"><br>
<input type="submit" value="Check">
</form>
</body>
</html>
<?
//LUHN ALGORITHM PHP CONVERSION
// CREDIT CARD NUMBER CHECKER BY JACK BELLAMY - JACKBELLAMY.CO.UK
//----------- RETREIVE THE STRING FROM THE FORM POST -------------//
$var = $_POST['cardnumber'];
//----------- SPLIT THE 16 CHARACTOR STRING INTO INDIVIDUAL CHARACTERS AND ASSIGN TO INDIVIDUAL VARIABLES ---------------//
$n0 = $var[0];
$n1 = $var[1];
$n2 = $var[2];
$n3 = $var[3];
$n4 = $var[4];
$n5 = $var[5];
$n6 = $var[6];
$n7 = $var[7];
$n8 = $var[8];
$n9 = $var[9];
$n10 = $var[10];
$n11 = $var[11];
$n12 = $var[12];
$n13 = $var[13];
$n14 = $var[14];
$n15 = $var[15];
// ---------------------CHECKING THE CARDNUMBER FORM POST SUBMITS ALL VALUES PROPERLY
/*
echo $n0;
echo $n1;
echo $n2;
echo $n3;
echo $n4;
echo $n5;
echo $n6;
echo $n7;
echo $n8;
echo $n9;
echo $n10;
echo $n11;
echo $n12;
echo $n13;
echo $n14;
echo $n15;
*/
//-------ASSIGNING THE NEW VARIABLE VALUES ------------//
$n14_new = ($n14*2);
$n12_new = ($n12*2);
$n10_new = ($n10*2);
$n8_new = ($n8*2);
$n6_new = ($n6*2);
$n4_new = ($n4*2);
$n2_new = ($n2*2);
$n0_new = ($n0*2);
//!-----!------!//
//------------------------TESTING WITH THE NEW VARAIBLE *2
/*
echo $n0_new;
echo $n1;
echo $n2_new;
echo $n3;
echo $n4_new;
echo $n5;
echo $n6_new;
echo $n7;
echo $n8_new;
echo $n9;
echo $n10_new;
echo $n11;
echo $n12_new;
echo $n13;
echo $n14_new;
echo $n15;
*/
//--------- THE MATHS FOR THE COMPLETE SUM ------------ //
$isitlegit = ($n0_new+$n1+$n2_new+$n3+$n4_new+$n5+$n6_new+$n7+$n8_new+$n9+$n10_new+$n11+$n12_new+$n13+$n14_new+$n15);
//!----MATHS-----!//
?>
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" type="text/css" href="veri-styles.css"
</head>
<body>
<div class="container">
<div class="main-wrapper">
<div id ="verification-wrapper">
<form action="" method="post" enctype="application/x-www-form-urlencoded">
<div id="input-holder">
<input name="cardnumber" type="text" class="input-style" placeholder=""> </input>
<button class="button-style">Verify my card</button>
</div>
</form>
<div class="result-class">
<?php
if(isset($isitlegit) && ($isitlegit % 5 == 0)) { // it's good
$new = "<img src='correct.jpg' />";
} elseif(isset($isitlegit) && ($isitlegit % 5 != 0)) { // it's bad
$new = "<img src='incorrect.jpg' />";
} elseif(!isset($isitlegit)) { // make sure this is not set
$new = "<img src='card-number-required.jpg' />";
}
echo $new;
?>
</div>
</div>
</div>
</body>
</html>
You have three states you need to account for, 'open', 'correct', and 'incorrect' To do that you start with an 'open' state.
$new = "<img src='please_enter your card number.jpg' />";
echo $new;
You would use that in the first condition of your test -
if(isset($var) && ($isitlegit % 5 == 0)) { // it's good
$new = "<img src='correct.jpg' />";
} elseif(isset($var) && ($isitlegit % 5 != 0)) { // it's bad
$new = "<img src='incorrect.jpg' />";
} else { // the default view
$new = "<img src='please_enter your card number.jpg' />";
}
echo $new;
Once submitted you can only have one of two states, never to return to the default.
In addition you have left your stylesheet declaration un-closed. Please change it to this -
<link rel="stylesheet" type="text/css" href="veri-styles.css" />
if(false===$submitted){
echo "Please type your card details here";
} else {
if($isitlegit % 5 == 0) {
$new = "<img src='correct.jpg' />";
echo $new;
}
else {
$new = "<img src='incorrect.jpg' />";
echo $new;
}
}
The best way I can think of to achieve this is with AJAX. Create a div on your page where your initial message is displayed, and then use an AJAX call to submit the form data to a script you'll use to validate it, and within that script, echo either a "Success! Your card has been verified" or a "Unable to validate your card" message. Then have your AJAX function change the .innerHTML of the div to the responseText element. You can see this answer for a good starting point, as well as a note about using the jQuery library to simplify your AJAX implementation.
Edit: In response to another user's downvote, here's some code.
First, let's give your HTML form a call to an AJAX object we'll create. So change your HTML form code to this:
<form onSubmit="return process()">
process() is a javaScript function we're going to set up later with the AJAX call. You already have a div established where you want this data to appear, but we need to give it an id, so change:
<div class="result-class">
please enter your card number<img src='correct.jpg' />
</div>
to this:
<div class="result-class" id="result">
Please enter your card number.
</div>
And also assign an id to your text input:
<input name="cardnumber" type="text" class="input-style" placeholder="" id="cardnumber"> </input>
Now we'll create the javaScript function. (This is easier with jQuery, but I wrote it in plain javaScript so you won't need the dependancy if you're not familiar with how to use jQuery.
function process(){
var xmlhttp = new XMLHttpRequest();
var cardnumber = document.getElementById("cardnumber").value;
var result = document.getElementById("result");
xmlhttp.onreadystatechange=function(){
if (xmlhttp.readyState==4 && xmlhttp.status==200){
result.innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","validate.php?cardnumber=" + cardnumber,true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send();
return false;
}
Now, use whatever php script you were going to use to validate the cc# and save it as a separate script called validate.php Use $_GET['cardnumber'] variable to fetch the cardnumber data sent from the form, run it through whatever validation process you are using. Use an if statement to create the responseText element for our AJAX function. You might have your script set some variable to boolean true or false to check this. I'll use $validated here.
if($validated){
echo 'Your card has been verified!';
} else {
echo 'Unable to validate your card.';
}
The AJAX function will change the text inside the "result" div to whatever text is echoed by your validate.php script.
This should get you on the right track, barring any syntax errors, I just wrote this all off the cuff.
I am new to the world of PHP and have put together a form that multiplies an entered value. However when I attempt to validate if a person has not entered any values to return an error message, it does display the message. My code below. Appreciate if you could also suggest improvements.
<?php
$counter = 0;
if(isset($_POST["submit"])) {
$start = $_POST["start"];
$end = $_POST["end"];
$multiply = $_POST["multiplication"];
// if($_POST["start"] == "" && $_POST["end"] == "" && $_POST["multiplication"] == "") {
// print "Please enter some values";
// }
if(!isset($_POST["start"], $_POST["end"], $_POST["multiplication"])) {
print "Please enter some values";
}
// for($start;$start<$end;$start++) {
// $counter = $counter +1;
// $multiplication = $counter * $multiply;
// print "$counter <br />";
// print "$counter multiplied by $multiply = $multiplication <br />";
// }
}
?>
<html>
<head>
<title>Sample Multiplication</title>
</head>
<body>
<form name="multiply" method="post" action="multiplication_sample.php">
<input type="text" name="start" value="<?php if(isset($_POST["start"])) { print $start; } ?>">
<input type="text" name="end" value="<?php if(isset($_POST["end"])) { print $end; } ?>">
<input type="text" name="multiplication" value="<?php if(isset($_POST["multiplication"])) { print $multiply; } ?>">
<input type="submit" name="submit" value="submit">
</form>
<?php
if(isset($_POST["submit"])) {
for($start;$start<$end;$start++) {
$counter = $counter + 1;
$multiplication = $counter * $multiply;
print "$counter multiplied by $multiply = $multiplication <br />";
}
}
?>
</body>
</html>
I think that isset will make sure a variable is not NULL, however, "blank" is not the same as null. If you submit a form with blank values, the variable is still being set, it is just empty.
When the form is submitted, the content of the input fields is sent to the server.
If those input fields are empty, the server gets an empty string for each input -- but it gets something ; so, the $_POST["start"], $_POST["end"], $_POST["multiplication"] items are set -- even if they only contain empty strings.
You could check :
If the fields contain an empty string : if ($_POST["start"] === '')
Or if if contains only blank spaces : if (trim($_POST["start"]) === '')
Or if they are empty : if (empty($_POST["start"]))
If the fields aren't defined your code will print your message in the html before the <html> tag appears. Most browsers won't display it or display it in an unexpected place.
You should move the message display somewhere in the html where the user could see it.
And as other pointed out, except on the first call of the page the fields will have an empty value but still exists (and so isset will return TRUE)
I hope, I understand you right. It is
if(!isset($_POST["start"], $_POST["end"], $_POST["multiplication"])) {
print "Please enter some values";
}
that works not as expected? It seems, that you assume an empty string means, that nothing is set, what is not true.
$x = "";
isset($x); // true
Use empty() or just $_POST['start'] == '' instead.