How can I find the ‘public absolute’ path of a file?

Short Version

I need to give in the absolute path of a Javascript file that is to be hooked into something. It works if I hardcode it into

‘http://mywebsite.com/folder/script.js’

However I prefer not to hardcode it in. I can do dirname(___FILE___), which gives me this:

‘/home/public_html/folder/script.js’

But that fails, the script isn’t loaded. What’s the best practice to find the absolute path (starting with ‘http://’)?

–

Long Version

On this page in the WordPress documentation relating to the tinyMCE plugin creation, I found this:

// Load the TinyMCE plugin : editor_plugin.js (wp2.5)
function add_myplugin_tinymce_plugin($plugin_array) {
   $plugin_array['myplugin'] = URLPATH.'tinymce/editor_plugin.js';
   return $plugin_array;
}

There’s a note at the bottom saying:

Note: when using the mce_external_plugins filter, the url should point
to the actual plugin file, as in the example above.

A bit higher in the documentation they say

mce_external_plugins: passes in/out an associative php array
‘plugin_name’ => ‘plugin_url’. The url should be absolute and on the
same domain as your WordPress installation.

I am completely puzzled because I can’t find any information about ‘URLPATH’ which is used in their example. I don’t think it exists in PHP, WP or TinyMCE, but I could be wrong.

Incidentally, were this to be an error in the WP documentation, what would I replace URLPATH with to make it work?