Debug ошибки при использовании AJAX, как?

if (isset($_POST['name'])) 
{
    $email_to = "**@**.com";
    $email_subject = "MESSAGE";
 
    function died($error) 
    {
        echo "Sorry, but there were error(s) found with the form you submitted. ";
        echo "These errors appear below.<br /><br />";
        echo $error."<br /><br />";
        echo "Please go back and fix these errors.<br /><br />";
        die();
    }
 
    
    if(!isset($_POST['name']) || !isset($_POST['mail']) || !isset($_POST['message'])) 
    {
 
        died('Sorry, but there appears to be a problem with the form you submitted.');       
    }
 
    $name = $_POST['name']; 
    $mail = $_POST['mail'];
    $message = $_POST['message'];
 
    $error_message = "";
    $string_exp = "/^[A-Za-z .'-]+$/";
    
    if (!preg_match($string_exp, $name)) 
    {
        $error_message .= 'The Name you entered does not appear to be valid.<br />';
    }
    if (strlen($message) < 2) 
    {
        $error_message .= 'The Comments you entered do not appear to be valid.<br />';
    }
    if (strlen($error_message) > 0) 
    {
        died($error_message);
    }
    $email_message = "Form details below.\n\n";
 
    function clean_string($string) 
    {
        $bad = array("content-type","bcc:","to:","cc:","href");
        return str_replace($bad,"",$string);
    }
 
    $email_message .= "Name: ".clean_string($name)."\n";
    $email_message .= "Mail: ".clean_string($mail)."\n";
    $email_message .= "Message: ".clean_string($message)."\n";
 
 
    // create email headers
    $headers = 'From: ' .$email_from. "\r\n" . 'Reply-To: '. $email_from."\r\n" .
                          'X-Mailer: PHP/' . phpversion();
    mail($email_to, $email_subject, $email_message, $headers);