I would also like to programmatically know the path of a file that has been uploaded to a repository.

My purpose is to use an image in the header of a PDF file that I generate, using the tcpdf library that comes with Moodle.

I can specify an image to put in the header, if that image is under the {moodle root dir}/pix directory.  For example:

require_once( $CFG->libdir . '/pdflib.php' );
...
$doc = new pdf;
$doc->setPrintHeader( true )
$doc->setHeaderData( 'pix/f/writer-48.png', 15, 'The title', 'The subtitle' );
...

But, what if I want to use an image that has been uploaded to, for example, the server repository?

I know the image is there.  How do I determine a path to that image file that the tcpdf library will accept?  I think that the file path I provide to TCPDF::setHeaderData() is supposed to be relative to the Moodle root directory.

Because I did not know any better, I started by reverse-engineering how the Essential theme prints the header image as specified by that theme plugin's configuration setting named "logo".  That led me to theme_config::setting_file_url() in moodle/lib/output.lib.

That particular setting_file_url() function does not directly help me with my problem, because I want to get the URL of an image file that is specified by a configuration setting in a local plugin of mine.  But, it did give me other crumbs to follow:  the moodle_url::make_file_url() and ::make_pluginfile_url() functions in moodle/lib/weblib.php.

I then read the documentation in https://docs.moodle.org/dev/File_API.

I still cannot get what I want to work.  But, the above crumbs may help somebody.  I know that I have drifted away from the scope of the original post.