Why is my form error message never displaying?

I have two pages apply.php and registration.php.

In registration.php, i have

if(isset($_POST['submitted'])){
 //validation part
 $form_error ='';
 if(!$fullname开发者_JS百科){
  $form_error.= "Enter full name<br />";
  header('Location: apply.php');

And in apply.php, I display an error message if an error happened:

<p><?php if(isset($form_error))echo $form_error?></p>
<form  action="registration.php" method="post">

<label for="fullname">Fullname</label>
<input type="text" name="fullname" />

Why I am not getting echoed error message "Enter full name" in apply.php ?

You're not getting it because apply.php doesn't know about $form_error - it was initialised in registration.php, but not in apply.php.

You could do the following:

$_SESSION['form_error'] = "Enter full name<br />";

Then you could access that on apply.php.

<p><?php if(isset($_SESSION['form_error']))echo $_SESSION['form_error']?></p>
<form  action="registration.php" method="post">

<label for="fullname">Fullname</label>
<input type="text" name="fullname" />

Alternatively, you could also pass the error through the header (via GET):

$form_error.= "Enter full name<br />";
header('Location: apply.php?form_error=' . urlencode($form_error));

And access it like this in apply.php:

<p><?php if(isset($_GET['form_error']))echo $_GET['form_error']?></p>
<form  action="registration.php" method="post">

<label for="fullname">Fullname</label>
<input type="text" name="fullname" />

Do not use header nor different files.
Put everything into one and on error just show the form. Redirect on success only.

Make it like this:

<?
if ($_SERVER['REQUEST_METHOD']=='POST') { 
 if(!$fullname) $form_error.= "Enter full name<br />";
 // other validations
 if (!$form_error) { 
    //writing to database
    Header("Location: ".$_SERVER['PHP_SELF']); 
    exit; 
  } 
}  
?>
<form>
...

See also http://en.wikipedia.org/wiki/Post/Redirect/Get