Formulaire de contact invalide

<?php
// ---------------------------------
// ENVOI du FORMULAIRE - ajax
// ---------------------------------
if( isset($_POST['first_name'], $_POST['last_name'], $_POST['email'], $_POST['comments']) )
{
	// -----------------
	// EDIT THE 2 LINES BELOW AS REQUIRED
	$email_to 			= "dm15517@gmail.com";		// receiver email
	$email_subject 		= "New Message Received";
	// -----------------
	// fonctions utiles
	if( !function_exists('show_msg_error') )
	{
		function show_msg_error($error)
		{
			// your error code can go here
			echo "<div style=\"margin:20px 0;padding:20px;background:#fff;color:red;border:1px solid red;border-radius:3px;\">";
			echo "<p>Sorry, but there were error(s) found:</p>";
			echo "<p>".$error."</p>";
			echo "<p>Please correct them and try again</p>";
			echo "</div>";
			exit;
		}
	}
	// ---------
	if( !function_exists('clean_string') )
	{
		function clean_string($string)
		{
			$bad = array("content-type","bcc:","to:","cc:","href");
			return str_replace($bad,"",$string);
		}
	}
	// -----------------
	// Récupération des champs du formulaire 
	$first_name			= ( !empty($_POST['first_name']) ) ? 	$_POST['first_name'] 	: ''; 	// required	
	$last_name			= ( !empty($_POST['last_name']) ) ? 	$_POST['last_name'] 	: ''; 	// required  
	$email 				= ( !empty($_POST['email']) ) ? 		$_POST['email'] 		: ''; 	// required	
	$comments			= ( !empty($_POST['comments']) ) ? 		$_POST['comments'] 		: ''; 	// required	
	// -----------------
	// Gestion des erreurs
	$error_message 		= '';
	// On vérifie que tous les champs obligatoires sont remplis
	// ---------
	if( empty($first_name) )
	{
		$error_message .= 'Your First Name is mandatory<br />';
	} 
	// ---------
	if( empty($last_name) )
	{
		$error_message .= 'Your Last Name is mandatory<br />';
	} 
	// ---------
	if( empty($email) )
	{
		$error_message .= 'Your Email address is mandatory<br />';
	} 
	elseif( !empty($email) && !filter_var($email, FILTER_VALIDATE_EMAIL) )
	{
		$error_message .= 'A valid Email address is mandatory<br />';
	}
	// ---------
	if( empty($comments) )
	{
		$error_message .= 'A Question or Comment is mandatory<br />';
	} 
	// -----------------
	// ERREUR ?
	if( !empty($error_message) )
	{
		show_msg_error($error_message);
	} 
	// -----------------
	// PAS d erreur : envoi par mail
	else {
		$email_from = $email;  
 
		$email_message = "Form details below.\n\n";	 
		$email_message .= "First Name: ".clean_string($first_name)."\n";
		$email_message .= "Last Name: ".clean_string($last_name)."\n";
		$email_message .= "Email: ".clean_string($email_from)."\n";	
		$email_message .= "Comments: ".clean_string($comments)."\n";
 
		// create email headers
		$headers = 'From: '.$email_from."\r\n".
		'Reply-To: '.$email_from."\r\n" .
		'X-Mailer: PHP/' . phpversion();
		if( mail($email_to, $email_subject, $email_message, $headers) )
		{
			echo "<div style=\"margin:20px 0;padding:20px;background:#fff;color:green;border:1px solid green;border-radius:3px;\">";
			echo "<p><b>Thank you. Your message has been correctly sent.</b></p>";
			echo "</div>";
			exit;
		} else {
			echo "<div style=\"margin:20px 0;padding:20px;background:#fff;color:red;border:1px solid red;border-radius:3px;\">";
			echo "<p>An error occured during sending.</p>";
			echo "<p>Please try again, or send it to: ".$email_to."</p>";
			echo "</div>";
			exit;
		}
	}
	// -----------------
}
// ---------------------------------