<?php

if(isset($_POST['submit'])) {

    $username = $_POST['username'];
    $password = $_POST['password'];

    if( $username == "admin" && $password == "admin" ) {
        echo "Logged in";
    } else {
        echo "Bitch!";
    }

}

<form method="post" action="<?=$_SERVER['PHP_SELF'];?>">
<input type="text" placeholder="Enter Username" name="username" required>
<input type="password" placeholder="Enter Password" name="password" required>
<input type="submit" name="submit"/>
</form>