Form is not navigating to another page - php

Question

<form id="enter" action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post" onsubmit="return validateForm(this);" >
<p>
<input id="submitBtn" name="submitDetails" type="submit" value="Submit Details" onClick="myClickHandler(); return false;" />
</p>
</form>
<script type="text/javascript">
function myClickHandler(){
if(validation()){
showConfirm();
}
}
</script>
<?php
session_start();
$outputDetails = "";
$outputDetails .= "<table id='sessionDetails' border='1'>
<tr>
<th>Number of Sessions:</th>
<th>{$_POST['sessionNum']}</th>
</tr>";
$outputDetails .= "</table>";
echo $outputDetails;
?>
Above is the code for my form. What I am trying to do is that if the user submits the form, then it will go back to its own page. But if the "SessionNum" equals '1', then instead of posting the form to itself, it should post the form or in other words navigate to the "session_marks.php' page but it is not idng this, if sessionNum equals 1 then it still submits form or navigate back to its own page, what am I doing wrong?
Also lets say it displays a number for the sessionNum and then I submit the form and it submits the form back to itself, the number disappears, how do I keep the number displayed when submitting the form to itself?
Thanks

Answer 1

Where is the conditional logic to change the target of the form post? All I see in the form tag is this:
action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>"
This will always set the form's action to be the current PHP file, not any other PHP file. If you want to conditionally post to a different file, you'll need to add conditional logic in there. Something like this (though there may be better ways to do it, keep in mind that I'm very out of practice with PHP):
action="<?php $_POST['sessionNum'] == 1 ? echo 'session_marks.php' : echo htmlentities($_SERVER['PHP_SELF']); ?>"
As for the number disappearing, I don't see any form element with the name sessionNum. If there isn't such a form element, then there will be nothing in $_POST['sessionNum'], so the number will "disappear" because there's no value to be displayed.

Answer 2

If the above is your actual code, session_start(); has to be placed before ANY other output (html, php's echo, print etc...)