<?php

$logged_in = false;

if(isset($_POST['username']) && isset($_POST['password'])){
    $username = $_POST['username'];
    $password = $_POST['password'];

    if($username === 'Ahmad' && $password === '123'){
        echo "Logged in !";
        $logged_in = true;
    }
    else
        echo "Worng Credentials !";

}

if(!$logged_in){
?>

<form action="" method="post">
    <input type="text" name="username" placeholder="Username">
    <input type="password" name="password" placeholder="Password">
    <input type="submit">
</form>

<?php }