Using require_once() within admin

Avoid native php functions

1. They are not relative, so breaking stuff is easy. (like in the OPs case)
2. Why role your own if core got a function for you?
3. They don’t consider edge cases

You should NEVER use the constants

• STYLESHEET_DIRECTORY
• TEMPLATE_DIRECTORY
• WP_PLUGIN_DIR
• etc…

^{Remember: Those are not API! and can therefore change!}

but INSTEAD USE the following API functions:

• Parent Theme: get_template_directory() / get_template_directory_uri()
• Child Theme: get_stylesheet_directory() / get_stylesheet_directory_uri()

You can use both in Plugins and Themes:

• plugin_dir_path(__FILE__).'/your/path/to/file.ext ^{*) for Themes also}
• plugin_dir_url( __FILE__ ).'/your/path/to/file.ext

but plugin_dir_url() is reserved only for Plugins.

Keep in mind…

That there’re

• Plugins
• MU-Plugins
• Drop-Ins
• Themes
• Themes in directories registered via register_theme_directory( $dir );

So please stick with the native API functions like wp_upload_dir();, content_url();, etc.

In general: Take a look at /wp-includes/link-template.php for a whole bunch of nice path and URl related API functions.