$url = "http://www.bufa.es";
function valida_url($url){  
    if (preg_match('/^(http|https|ftp):\/\/([\w]*)\.([\w]*)\.(com|net|org|biz|info|mobi|us|cc|bz|tv|ws|name|co|me)(\.[a-z]{1,3})?\z/i', $url)) {
        echo 'dominio correcto';
    }else{  
        echo 'dominio incorrecto';
    }
}
// Modo de uso
valida_url($url);

function valida_email($email, $mx = false){  
  if(eregi("^([_a-z0-9-]+)(\.[_a-z0-9-]+)*@([a-z0-9-]+)(\.[a-z0-9-]+)*(\.[a-z]{2,4})$", $email))  
    if($mx){  
      list($username, $domain) = split("@", $email);  
      return getmxrr($domain, $mxrecords);  
    }else  
      return true;  
    else  
  return false;  
}