Redirect back into wordpress structure.

Coding a plugin.
In one particular instance, I call a php script for processing information. One of the options based on the processed info is redirecting to a specific plugin screen.

I have this;
$redirecturl = WP_PLUGIN_URL() . ‘php/manage_pas.php&careturn=1’;
header(“Location: ” . $redirecturl);

The problem is that because the file isn’t a viewable page in the lug, it doesn’t have access to all the WP functions. While I do use “global wpdb” for the DB functions, I don’t know of any way to pull in that WP_PLUGIN_URL function.