Create a WordPress administrator user account via PHP/FTP.

init_create_my_administrator_user.php

<?php
/*
  Sometimes you have FTP access to a site, but don't have a WP administrator account.
  You can use this code to create one.
  
  1. Add this code into any plugin or theme file that you know will be run.
  2. Visit /?createmyadministratoruser=1.
  3. Delete the code you added.
*/
function init_create_my_administrator_user() {
	if(!empty($_REQUEST['createmyadinistratoruser'])) {
		wp_insert_user(array(
			'user_login' => 'tempadmin',      //change this
			'user_pass' => 'temppass123',     //change this
			'user_email' => '[email protected]',  //change this
			'role' => 'administrator',
		));
		
		echo "Admin user created. Remember to delete the code you added for this.";
		exit;
	}
}
add_action('init', 'init_create_my_administrator_user');