It seems that if you have a .php file inside a folder, then it gets its own error_log file, meaning all this time I've been having a little error log grow inside an old abandoned folder, because I forgot it had a broken .php file in there!

.. Oops!!

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On my previous server, I had error messages pop up directly in the output, so I've never really had to deal with error_log files before.
Somewhat dodgy, I know, but I trusted my code to be able to cope well with things.
But on this occasion, the php script was outputting an image, so it never had a chance to go "Hey, ERROR!!"
Now that I'm on the new server, with errors outputting to error_log files, I assumed everything was being placed onto a single error_log at the root of the server. But.. Nope!
Today, during a "Bulk/Everything Backup", I noticed a 5Mb error_log displaying errors dating back to the 10th of April, which was being generated, with the following error.

"imagecolordeallocate() expects exactly 2 parameters, 1 given"

Which begs the question.. How long, exactly, has that error been around!?

Aw well, shouldn't be too much of a fix.