PHP E-Mail Advanced Validation

<?php
/**
 * PHP E-Mail Advanced Validation
 *
 * Coded by Christian Ditaputratama < ditatompel [at] gmail [dot] com >
 *
 * This program used to check whether e-mail address is valid.
 * The validation start from email address format, MX record, and SMTP
 * mailbox check.
 *
 * Please note that some mail server accept all mail although the mailbox
 * recipient does not exists!
 *
 * LICENSE :
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 as published by
 * the Free Software Foundation.
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
 * more details.
 *
 * Usage :
 * check_email($email); // for email format and MX record validation.
 * check_email($email, FALSE); // for email format validation only.
 * check_email($email, TRUE, TRUE); // for email format, MX record and SMTP
 * Mailbox validation.
 */

// Function to get SMTP respond code
function serverRespond($respond) {
    return substr($respond, 0, 3);
}

// function to check whether mailbox is exists
// return bool
function checkSMTP($smtpHost, $emailAddr) {
    if ( !($socket = fsockopen($smtpHost, 25, $errno, $errstr, 15)) )
        return FALSE;
    if ( serverRespond(fgets($socket, 256)) != '220' )
        return FALSE;
    fwrite($socket, 'HELO ' . $smtpHost . "\r\n");
    if ( serverRespond(fgets($socket, 256)) != '250' )
        return FALSE;
    fwrite($socket, 'MAIL FROM: <mailvalidator@devilzc0de.org>' . "\r\n");
    if ( serverRespond(fgets($socket, 256)) != '250' )
        return FALSE;
    fwrite($socket, 'RCPT TO: <' . $emailAddr . '>'."\r\n");
    if ( serverRespond(fgets($socket, 256)) != '250' )
        return FALSE;
    fwrite($socket, 'QUIT' . "\r\n");
    fclose($socket);
    return TRUE;
}

// main email validation function
// return bool
function check_email($email, $cekMXRecord=1, $checkSMTPBox=0) {
    // check if the email address format is valid.
    if( !preg_match('/^\w[-.\w]*@(\w[-._\w]*\.[a-zA-Z]{2,}.*)$/', $email, $domain) ) {
        return FALSE;
    }

    // check MX DNS record if $cekMXRecord set to 1
    if ( $cekMXRecord && function_exists('checkdnsrr') ) {
        if( !checkdnsrr($domain[1] . '.', 'MX') )
            return FALSE;

        // Check SMTP Mailbox is exists if $checkSMTPBox set to 1
        if ( $checkSMTPBox && function_exists('dns_get_record') ) {
            $smtpTarget = dns_get_record($domain[1], DNS_MX);
            return checkSMTP($smtpTarget[0]['target'], $email);
        }
    }
    return TRUE;
}
?>