<form method="post">
<p>Username: <input type="text" name="usr"></p>
<p>Password: <input type="text" name="passwd"></p>
<p><input type="submit" value="Login"></p>
</form>

// After checking if the request is POST...
if($_POST["usr"]=="myuser"&&$_POST["passwd"]=="mypassword") {
     // Set the cookie and go to admin page...
} else {
     // Show login error...
}