WordPress functionality not working in custom TinyMCE editor buttons

I created a simple TinyMCE plugin as shown below. Basically, this is the new button in the TinyMCE editor that when pressed brings up a lightbox (thick frame).

// create plugin
tinymce.create('tinymce.plugins.callthickbox', {  
    init : function(ed, url) {  
        ed.addButton('callthickbox', {  
            title   : 'This is Thick Box',  
            cmd     : 'showthickbox'
        });
        ed.addCommand('showthickbox', function() {
            ed.execCommand('mceInsertContent', false, callthatthickbox(url,null));
        });

    //blah blah blah the rest...
});
tinymce.PluginManager.add('callthickbox', tinymce.plugins.callthickbox);

and call it with:

// call the PHP form
function callthatthickbox(url) {
    tb_show( "This is Thick Box", url + '/thickbox-form.php', false);   
}

The thick box will pop up and the PHP files (the content of the thick box) will work fine, but all WP functions will not work in these PHP files, such as wp_list_categories() or bloginfo('url') .

Apparently the PHP file lost access to WP functions. I could try passing variables through the URL, but that doesn't seem like a good solution, especially if I want to pass arrays.

Does anyone have a better solution?

Thanks a lot before!

EDIT: I've tried to load wp-load.php into a PHP file and it works fine, but isn't that a bit overkill? Any ideas?