How to send a form only if a field is empty - php

Question

Being a total newbie in PHP, i have issues with implementing the Google reCaptcha on my website.
I oriented myself towards an easier solution which would be: if this field is empty, SEND the form.
Here is my code:
<form method="POST" action="/#contact">
<!-- Name + mail -->
<input type="text" required placeholder="Name" name="name" />
<input type="email" required placeholder="Email" name="email" id="email" value=""/>
<select name="subject" id="category">
<option value="">Subject</option>
<option value="1">1</option>
<option value="1">2</option>
<option value="1">3</option>
<option value="1">4</option>
</select>
<input id="submit" name="submit" type="submit" value="Send"/>
<input type="reset" value="Clear" class="alt" />
<div>
<label>If you're human, leave this field epmty</label>
<input type="text" name="comment">
<?php
if(empty($_POST['comment'])) {
// the field is empty
}
?>
</div>
</form>
My problem is: the form is still being sent, even there is something in the field.
Thanks in advance for your help.

Answer 1

There are 2 solutions to this:
Call a php file that validates the inputs. When a user hits the send button, it goes to a php page. This page retrieves the values of the filled in form and validates it. If an invalid input is found, you can call the main page (ex. index.php) with an error parameter in the url (ex. index.php?error=botfound). And in the beginning of your index.php file, you can write a check if there is an error parameter and display the error message if you want.
Let javascript validate the values of the inputs before the values are sent to a php file. To do this, you change your code from <form method="POST" action="/#contact">
to:
<form method="POST" action="/#contact" onsubmit="DoSubmit();">
With a javascript function something like:
<script>
function DoSubmit(){
if(document.getElementById('idOfInput').value != ""){
alert("Wrong input"); //Or you can actually add an error message to your page.
return false;
}
return true;
}
</script>
I think that script will make sure if the value of the input with id 'ifOfInput' is not empty, it'll give an alert and will not call the php script.
(By the way, you can also make that input invisible so it doesn't annoy normal users but bots will still fill in the input.)
Hope this helped :)