Show dialog box in TinyMCE plugin and use WordPress php functions in it

Instead of requireing admin.php you just can use WP built-in ajax functionality, even if it’s not ajax in this case.

Add a hook

 add_action('wp_ajax_my_plugin_function', 'my_plugin_function_callback');

Create your output function (callback)

function my_plugin_function_callback() {
    // do stuff
}

Call it this way

instead of:

file : url + '/box.php',

you should be able to use the global ajaxurl:

file: ajaxurl + '?action=my_plugin_function&optional=data ...

Maybe add a nonce for security reasons, but that’s up to you 🙂