/**
 * 隐藏部分字符串后的邮箱地址 
 * @param string $mail 
 * @return string 
 */
function hidden_mail($mail){    
    if (!preg_match("/^[0-9a-zA-Z]+@(([0-9a-zA-Z]+)[.])+[a-z]{2,30}$/i", $mail)) {        return '';    
    }    
    $email_array = explode("@", $mail);    
    $prefix = (strlen($email_array[0]) < 3) ? $email_array[0] : substr($mail, 0, 2);    
    $mail = preg_replace('/([\d\w+_-]{0,100})@/', '***@', $mail, -1);    
    return  $prefix.$mail;
}