On error - retain other field information

Ajax could help here - have the submit button run a js then a server side check before sending the form.
Otherwise if your form handling code is in the same page as the form - repopulate the fields with data from the $_POST variables.
If the form handling code is in a different page (recommended), then you could use a variety of methods to repopulate the fields: $_SESSION or $_COOKIE variables. I'd use $_SESSION myself, but up to you. Once the form data has been repopulated, you need to kill the particular $_SESSION variables.

e.g.

[B]formhandler.php[/B]
<?php
session_start();
if(isset($_POST['regformsubmit'])){
   ...(do form handling and validation - e.g. check for image)...
   if($form_error){ //if an error
       $_SESSION['regform']['username'] = stripslashes($_POST['username']);
       ...(etc - all fields)...
       header("Location:form.php");
       exit;
   }
}
?>

[B]form.php[/B]
<?php
session_start();
if(isset($_SESSION['regform'])){
   $username = $_SESSION['regform']['username']; 
   ...(etc)...
   unset($_SESSION['regform']);
}
?>

<form method="post" action="formhandler.php" ...>
   <label for="username">Username:</label>
   <input type="text" id="username" name="username" value="<?php echo $username;?>" />
  ...(etc)...
   <input type="submit" id="regformsubmit" name="regformsubmit" value="Register" />
 </form>

Hi Ardav many thanks for your reply,

im pretty new to php and web-design,

the form_reg.php calls form_vallidation.php and returns any errors found back to form_reg at the top of the form.

I will have a go at your solution and reply back. Iv not yet coded with js, but i am keen to learn new technologies,

Many thanks
Lloyd