If you readfile() a large file, then cancel the stream, the entire file ends up in the error_log.  For some reason, on stream cancel, it then writes the file into Apache's error_log.

Simple script:

header("Content-type: audio/mpeg");
header("Content-length: $song[size]");
@readfile($song[filename]);

Just start this, cancel the stream, and watch your error_log.

I tried manipulating php.ini's error log attributes, but it didn't help.  I turned off all logging, and it still showed up.

It's important to note it's not only readfile() that can place its contents into error_log.  I see this with other large (very large) outputs of just text.  If I cancel the downloading of a very large result page from PHP, it too will end up in the error_log.