Empty data added to text file in PHP - php

Question

I am doing a school project which require me to create a buy & sell website.
I have a form for user to input values such as name, email, etc. After it is filled up, they will press "submit" button, and the values will be written to my txt file.
However, I realized that, empty data are being written into the txt file as well, causing my text file to look like this
It happens when user click on the navigation button to access the form. Even when no values are entered. Empty spaces are written to my txt just by accessing the form.
Below is my code.
<?php
// define variables and set to empty values
$sellerNameErr = $emailErr = "";
$sellerName = $email = "";
if ($_SERVER["REQUEST_METHOD"] == "POST") {
if (empty($_POST["sellerName"])) {
$sellerNameErr = "Name is required";
}else {
$sellerName = test_input($_POST["sellerName"]);
}
if (empty($_POST["email"])) {
$emailErr = "Email is required";
}else {
$email = test_input($_POST["email"]);
// check if e-mail address is well-formed
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
$emailErr = "Invalid email format";
}
}
}
function test_input($data) {
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
?>
<form name="sell" action="" method="post">
<p><span class = "error">* required field.</span></p>
<p>Seller Name: <input type="text" name="sellerName" />
<span class = "error">* <?php echo $sellerNameErr;?></span></p>
<p>Email: <input type="text" name="email" />
<span class = "error">* <?php echo $emailErr;?></span></p>
<input type="submit" name="Submit" value="Submit" />
<?php
$fp=fopen("buynsell.txt", "ab");
fwrite($fp,"$sellerName\t $email\r\n");
fclose($fp);
?>
</form>
How can I prevent empty data from being added to my log file?

Answer 1

Before writing to the file, you need to verify that $sellerName and $email have values and that there were no errors.
if ($sellerName && $email && !$sellerNameErr && !$emailErr) {
$fp=fopen("buynsell.txt", "ab");
fwrite($fp,"$sellerName\t $email\r\n");
fclose($fp);
}
The way you have it written now you don't need to do anything to check that those variables are set because you're initializing them all to empty strings, but if you change that, you should use empty()/!empty() instead to avoid undefined variable notices.
If you move the file writing PHP code inside the post handling code at the top so that it will only run when the form has been posted, after the validation code that checks if the inputs are empty, then you should be able to just check for errors only. if (!$sellerNameErr && !$emailErr) { .... Personally, I think it makes more sense for it to be there anyway. It's easier to read if all your PHP code is together, and there's no need for that code to be down there in the form.

Answer 2

Mark, there are many ways to prevent that empty data does not get into your business logic.
As for your project the most simple way to achieve this would be to set an additional attribute within your html markup.
As for HTML5 this could by your first solution:
<input type="text" name="usrname" required>
https://www.w3schools.com/tags/att_input_required.asp
Not that really efficient as only on client side, so you should also implement this on your server side.
empty($_POST['user']) will not work as you are never posting that value, therefore you must add an value attribute to your input field:
<input type="text" name="user" value="" />