salam,
if(isset($_POST["button"])){ #isset
$login=$_POST["login"];
$pass=$_POST["pass" ];
$req=$conn->query("select pass from users where login='$login'")->fetch_assoc();
if(password_verify($pass,$req["pass"])){
echo "ok";
exit;
}
else {
echo "sehv oldu";
}
} #isset
echo...