if(mysql_query("запрос")!=0) echo "есть"; else echo "нет";
<form name="test" action="index.php" method="post"> <input type="text" name="login" value="" /> <? $login = $_POST['login']; $query = "SELECT * FROM `users`"; $result = mysql_query($query); if(mysql_query("$login")!=0) echo "есть"; else echo "нет"; ?>