Draft 3 of IEEE 1003.1 200x, "2.2 The Compilation Environment"
All identifiers that begin with an underscore and either an uppercase
letter or another underscore are always reserved for any use by the
implementation.
There are 2-3 third-party libs in PHP which use DEBUG and I left them
this way (primary example is pcrelib).
- Please test things and Andrei, can you check that pcrelib stopped
printing the warning messages?
