stderr error log from launchd scripts filled with non-errors
There is nothing wrong with the output you are capturing.

Convention Only

Developers are free to output anything they want to stdout and stderr. Convention suggests passing error related output to stderr but nothing enforces this choice.

In this instance the php script authors have written debug or progress information to stderr. I presume they expected stderr to be ignored, until a problem needs debugging.

See Confused about stdin, stdout and stderr? for a detailed question and answer on the subject.