Can't figure out why isset is not working - php

Question

I have a button that I need to call a function on when it is clicked but for some reason, the isset function is not seeing the button as being clicked.
Below are the two lines in the php/html file:
<input type="submit" id='details' name='details' class="pull-right btn-sm btn-primary" value='Show Details'/>
<h4><?php echo $subject_details; ?></h4>
The below is in an external. I know it is included because if I add an else to the if statement, and set that variable to something different, it will change.
$subject_details = '';
if(isset($_POST['details'])){
$subject_details = 'button clicked';
setDetails();
}
Thanks to anyone looking at this!

Answer 1

The isset() function only works for input type submit. What you can do is embed the button in a form and let the page reload to itself to grab the variable stored in an external php. As an example:
//in your main.php
<?php
include 'external.php';
?>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
<input type="submit" name="submit" value="Send">
</form>
<h4><?php echo $subject_details; ?></h4>
//in your external.php
<?php
$subject_details = '';
if(isset($_POST['submit'])){
$subject_details = 'button clicked';
}
?>

Answer 2

Without seeing the full code example, I can't be sure, but it's possible you're not specifying the method="post" attribute on the <form> tag. A form defaults to method="get" therefore you would need to use isset($_GET['details']) instead.
A working example would be:
<form action="" method="post">
<input type="submit" name='details' value='Show Details'>
</form>
<?php
if (isset($_POST['details'])) {
echo $_POST['details'];
} else {
echo 'No post data submitted';
}

Answer 3

you could play around with this... might help
<!-- head section -->
<script>
window.onload=function(){
var oCol=document.querySelectorAll('input[type="button"]');
for( var b in oCol ) if( b && oCol[b] && oCol[b].nodeType==1 ) oCol[ b ].onclick=function(e){
alert( this.tagName + ' '+this.id+' was clicked...' );
}.bind( oCol[b]);
};
</script>
<input type='button' id='b1' value='Button 1' />
<input type='button' id='b2' value='Button 2' />
<input type='button' id='b3' value='Button 3' />
<input type='button' id='b4' value='Button 4' />