What's wrong with my simple captcha php script?

I also have session_start(); at the top of my index page, its not working!

  <?php

    if(isset($_REQUEST['Submit'])){
      $key=substr($_SESSION['key'],0,5);
      $captcha = $_REQUEST['captcha'];
      if($captcha=!$key){
        exit();
    }
        $EmailFrom = "****";
        $EmailTo = "****";
        $Subject = "****";
        $contactname = Trim(stripslashes($_POST['contactname'])); 
        $companyname = Trim(stripslashes($_POST['companyname']));
        $username = Trim(stripslashes($_POST['username'])); 
        $phone = Trim(stripslashes($_POST['phone'])); 
        $email = Trim(stripslashes($_POST['email'])); 

        // validation
        $validationOK=true;
        if (!$validationOK) {
          print "<meta http-equiv=\"refresh\" content=\"0;URL=../404.php/\">";
          exit;
        }

        // prepare email body text
        $Body = "";
        $Body .= "Contact Name: ";
        $Body .= $contactname;
        $Body .= 开发者_如何学运维"\n";
        $Body .= "Company Name: ";
        $Body .= $companyname;
        $Body .= "\n";
        $Body .= "Preferred Username: ";
        $Body .= $username;
        $Body .= "\n";
        $Body .= "Phone: ";
        $Body .= $phone;
        $Body .= "\n";
        $Body .= "Email: ";
        $Body .= $email;
        $Body .= "\n";

        // send email 
        $success = mail($EmailTo, $Subject, $Body, "From: <$EmailFrom>");

        // redirect to success page 
        if ($success){
          print '<script type="text/javascript">';
          print 'alert("Your Submission will be reviewed by an Admin and you will receive an email shortly")';
          print '</script>';
          print "<meta http-equiv=\"refresh\" content=\"0;URL=http://****.com/****/\">";
        }
        else{
          print "<meta http-equiv=\"refresh\" content=\"0;URL=../404.php/\">";
        }
    ?>

You are missing a } at the end (before ?>)

Be sure to indent your code correctly to easily catch these errors.

Add another curly bracket at the end and it should be ok i think

You are not closing the curly bracket for your top level if statement if(isset($_REQUEST['Submit'])){

OR based on how you want your flow you can remove the curly bracket from

if($captcha=!$key){
    exit();

So that it becomes

if($captcha!=$key)
    exit();

Notice that i changed =! to != as well