headers_sent(&$filename, &$linenum) returns true and yet doesn't set either filename or linenum

From manual page: https://php.net/function.headers-sent

It says:

If the optional filename and line parameters are set, headers_sent() will put the PHP source file name and line number where output started in the filename and line variables.

Therefore, if I call headers_sent($filename, $linenum) and pass it both arguments, and it returns true, I expect it to set both filename and linenum to the file and line number where the output started.

And yet, I (pretty often) observe cases where it returns true and yet doesn't set filename and line number. I don't know how to reproduce, because, because of the very fact that it doesn't tell me where the output started, I have been unable to debug it. Otherwise I would be happy to provide example code that reproduces.

If there are legitimate cases for headers_sent() being unable to tell the file name and line number where the output started, then these cases should be documented in excruciating detail.

But I can't think of any such legitimate cases. My script is initiated by a php file, so wherever the output started, that "somewhere" MUST be at least backtraceable to some filename and line number (if it was started by eval()'d code, then it's the filename and line number that called eval(), that is given that you decided that only a filename and line number are to be returned and not a backtrace of any sort).

Start with the headers_sent() manual page linked in the issue and review its current description of the optional filename and line parameters. Investigate the reported cases where the function returns true without populating them, then document confirmed limitations or correct the wording. Done means the manual page explains the behavior accurately and includes reproducible guidance where possible.