valid_email.php

<?php

/**
 * Return true if the email address is valid.
 * From http://www.linuxjournal.com/article/9585.
 *
 * @return bool
 */
function isValidEmail($email) {
    $isValid = true;
    $atIndex = strrpos($email, '@');
    if ($atIndex === false) {
        return false;
    }
    $domain = substr($email, $atIndex+1);
    $local = substr($email, 0, $atIndex);
    $localLength = strlen($local);
    $domainLength = strlen($domain);
    if ($localLength < 1 || $localLength > 64) {
        // local part length exceeded
        return false;
    } else if ($domainLength < 1 || $domainLength > 255) {
        // domain part length exceeded
        return false;
    } else if ($local[0] == '.' || $local[$localLength - 1] == '.') {
        // local part starts or ends with '.'
        return false;
    } else if (preg_match('/\\.\\./', $local)) {
        // local part has two consecutive dots
        return false;
    } else if (!preg_match('/^[A-Za-z0-9\\-\\.]+$/', $domain)) {
        // character not valid in domain part
        return false;
    } else if (preg_match('/\\.\\./', $domain)) {
        // domain part has two consecutive dots
        return false;
    } else if (!preg_match(
        '/^(\\\\.|[A-Za-z0-9!#%&`_=\\/$\'*+?^{}|~.-])+$/',
        str_replace('\\\\', '', $local))) {
        // character not valid in local part unless 
        // local part is quoted
        if (!preg_match(
            '/^"(\\\\"|[^"])+"$/',
            str_replace('\\\\', '', $local))) {
            return false;
        }
    }
    // checkdnsrr is not implemented on Windows
    if (function_exists('checkdnsrr') && !(checkdnsrr($domain, 'MX') || checkdnsrr($domain, 'A'))) {
        // domain not found in DNS
        return false;
    }
    return true;
}