Retrieve admin.php Path, Not URL

Hi,

I’m in the middle of writing a plugin that needs to inlude admin.php. How can I retrieve the path for admin.php in the wp-admin directory? I mean not url but the installed directory path. I searched the forum and only could find the function, admin_url() http://codex.ww.wp.xz.cn/Function_Reference/admin_url.

But this is not what I’m looking for since some users might be changing the file location for security and using redirects.

Thanks for your information.