How to make a volunteering calculator in my html [closed] - php

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I'm currently coding a website in html and CSS. I have learned, php well at least enough for the thing i'm currenly trying to make. So I basically have the following,
How do you accomplish getting a number/constant 40 and subtracting that from a user inputted number. In a html document
<form action="welcome.php" method="post">
$string = "cool";
Hours:<input type ="text" name ="name"<br>
$string = "cool"
Hours: <input type="text" name="name"><br>
<input type="submit">
echo 40-"$cool";
</form>
this is wrong and will return the echo and original php code. If i wrap it in php it will display an error

Answer 1

The $cool should not be quoted. Also, why are you setting the $string two times with same value?
<form action="welcome.php" method="post">
<?php $string = "cool"; ?>
Hours:<input type ="text" name ="name"><br>
Hours: <input type="text" name="name"><br>
<input type="submit">
<?php echo 40-$cool; ?>
</form>
Also, the file itself has to be php, not html.

Answer 2

Firstly, you're injecting PHP inside HTML, you can't do that. It will produce a parse error, if your file is indeed a .php extension.
Edit: As noted in comments, you can inject PHP inside HTML, just as long as you include PHP tags and enclosed using <?php ?> or <?= ?>; the latter being short open tag syntax.
Now, as I understand it, you want another field where a user will enter a number, then substract (from user input) your number 40 being a constant, and add on the word "cool" after the result.
Here's how and with the user's name echo'd also if filled and checking if it's a number using is_numeric().
Sidenote: Using the number 2 in the input and with the name John, will output "38 cool John".
<?php
if(isset($_POST['submit'])){
if(isset($_POST['number']) && is_numeric($_POST['number'])){
$string = "cool";
$name = $_POST['name'];
$number = $_POST['number'];
$constant = 40;
$total = $constant - $number;
echo "$total $string $name";
} // brace for if(isset($_POST['number'])...
else{
echo "It's not a number";
}
} // brace for if(isset($_POST['submit']))
?>
<form action="" method="post">
Name:<input type ="text" name ="name"<br>
Number: <input type="text" name="number"><br>
<input type="submit" name="submit" value="Submit">
</form>
Footnotes:
You can also use an alternate method to echo the variables, such as:
echo "$total $string $name"; with a space between
echo "$total-$string-$name"; with a hyphen seperator
as noted in comments
The dot concatenation is just force of habit on my part.
Sidenote: You must use double quotes for this, since variables only get parsed inside double quotes.
Otherwise, you would get the following parse error using echo $total $string $name;:
Parse error: syntax error, unexpected '$string' (T_VARIABLE), expecting ',' or ';'...
Using echo $total-$string-$name; would work, but it will only echo the value from the substraction, instead of the intended echo'd string(s).

Answer 3

You don't need PHP for this at all. You can just use some basic java script. To use php, you need to set this whole page up differently.