Form validation and redirection

[captain_caveman]

Hi,

I have a problem with the following script that I'm using on a form page:

<?
function check_telephone($telephone){
if(!preg_match("/[^0-9\ ]+$/",$telephone)) return true;
else return false;
}

function check_postcode($postcode){
if(!preg_match("^[ .a-zA-Z0-9:-]{1,150}$",$postcode)) return true;
else return false;
} 

function check_email_address($emailaddress) {
  // First, we check that there's one @ symbol, and that the lengths are right
  if (!ereg("^[^@]{1,64}@[^@]{1,255}$", $emailaddress)) {
    // Email invalid because wrong number of characters in one section, or wrong number of @ symbols.
    return false;
  }
  // Split it into sections to make life easier
  $email_array = explode("@", $emailaddress);
  $local_array = explode(".", $email_array[0]);
  for ($i = 0; $i < sizeof($local_array); $i++) {
     if (!ereg("^(([A-Za-z0-9!#$%&'*+/=?^_`{|}~-][A-Za-z0-9!#$%&'*+/=?^_`{|}~\.-]{0,63})|(\"[^(\\|\")]{0,62}\"))$", $local_array[$i])) {
      return false;
    }
  }  

  if (!ereg("^\[?[0-9\.]+\]?$", $email_array[1])) { // Check if domain is IP. If not, it should be valid domain name
    $domain_array = explode(".", $email_array[1]);
    if (sizeof($domain_array) < 2) {
        return false; // Not enough parts to domain
    }
    for ($i = 0; $i < sizeof($domain_array); $i++) {
      if (!ereg("^(([A-Za-z0-9][A-Za-z0-9-]{0,61}[A-Za-z0-9])|([A-Za-z0-9]+))$", $domain_array[$i])) {
        return false;
      }
    }
  }
  return true;
}


// only validate form when form is submitted
if(isset($submit_button)){
	$error_msg='';

if($checkbox!=="checkbox"){
	$error_msg.="You must agree to the Terms and Conditions<br />";
}

if($title=='') {
	$error_msg.='Please enter a Title<br />';
}
if($first_name=='') {
	$error_msg.='Please enter your First Name<br />';
}
if($last_name=='') {
	$error_msg.='Please enter your Last Name<br />';
}
if($address_1=='') {
	$error_msg.='Please enter the first line of your Address<br />';
}
if($postcode==''){
	$error_msg.='Please enter your Postcode<br />';
	}
else if (!check_postcode($postcode)){
	$error_msg.='Please enter a valid Postcode<br />';
	}

if($telephone=='') {
	$error_msg.='Please enter your Telephone Number<br />';
	}
else if(!check_telephone($telephone)){
	$error_msg.='Please enter a valid Telephone Number<br />';
	}

if($email=='') {
  $error_msg.='Please enter your Email Address<br />';
  }
else if(!check_email_address($email)) {

  $error_msg.='Please enter a valid Email Address<br />';
}

// display error message if any, if not, proceed to other processing
if($error_msg==''){
	$email = $HTTP_POST_VARS[email];
$mailto = "[e-mail]";
$mailsubj = "Download Form Details";
$mailhead = "From: $email\n";
reset ($HTTP_POST_VARS);
$mailbody = "Values submitted from web site download form:\n\n";
$redirect = "index.html"; 
while (list ($key, $val) = each ($HTTP_POST_VARS)) { $mailbody .= "$key : $val\n"; }
if (!eregi("\n",$HTTP_POST_VARS[email])) { 
	mail($mailto, $mailsubj, $mailbody, $mailhead);
	header("Location: thankyou.html"); 
	exit; }
	} 
}
?>

The following code is then placed above the form:

<? if (!$error_msg=='') echo "<font color=red><p><strong>Please correct the folllowing errors before submitting form:</strong></p><p>$error_msg</p></font>" ?>

The form action returns it back to this page.

My main problem is that whilst it emails the details absolutely fine I cannot get it to redirect to a new page (thankyou.html) and I keep getting a warning about the line on which the following code resides

 header("Location: thankyou.html");

It says: "Cannot modify header information - headers already sent on "(and then references the line on which the above code is)

Can anybody see what is wrong?

Any help much appreciated.

[captain_caveman]

Not to worry I've just realised it was because I was trying to redirect to an html page , changed it to thankyou.php and it worked fine.