<?php
$login = $_POST['login'];
$parol = $_POST['parol'];
if ($login == "admin" && $parol == "admin") {
    echo 'Login parol to\'g\'ri kiritilsa bu matn ko\'rinadi';
} else {
    if (isset($_POST)) { echo '
        <form method="POST" action="bu_kod_yozilgan_sahifa.php">
            login<br /><input type="text" name="login"></input><br/>
            parol<br /><input type="password" name="parol"></input><br/>
            <input type="submit" name="submit" value="Kirish"></input>
        </form>';
    }
}
?>