A PHP file is being included from somewhere other than my application folder. How can I find it?

I'm trying to maintain a legacy application which has a real mess of calls like

include_once("config.php");

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The trouble is that a config.php file is being loaded from somewhere outside of my application folder, and returning an error, and exiting. This will be because PHP has a path setting allowing it to pick up files from somewhere else if they aren't found locally.

Is there a log of which files are being loaded when my application is running, or some other way of tracking down the location of that file?

Thanks