Redundant email checking in PHP
Like so:
Here you can see that is easier to have errors as an array, use alternative control statments to separate php from html:

 &lt;?php
    $messages = array();
    if(strlen($display)&lt;4)
    {
        $messages[] = 'Display name is too short';
    }
    if (empty($email))
    {
        $messages[] ='You did not put a valid email address. We require this to verify your account.';
    }   
    if(mysql_num_rows($check_display) != 0)
    {
        $messages[] = 'Display name is already in use';
    }

    if(mysql_num_rows($check_user) != 0)
    {
        $messages[] = 'Username is already in use.';
    }

    if(mysql_num_rows($check_mail) != 0)
    {
        $messages[] = 'Email is already in use.';
    }

    if ($pass != $pass2) 
    { 
        echo 'Passwords do not match.';  
    }
    ?&gt;
    &lt;div class="error_message"&gt;
        &lt;?php if(!empty($messages):?&gt;
            &lt;ol&gt;
            &lt;?php foreach($messages as $message):?&gt;
                &lt;li&gt;&lt;?php echo $message;?&gt;&lt;/li&gt;
            &lt;?php endforeach;?&gt;
            &lt;/ol&gt;
        &lt;?php endif;?&gt;
    &lt;/div&gt;