With MDL-39474 some changes about how to detect DEBUG_DEVELOPER were performed.

So, for 2.6 and upwards:

• Warnings:
  • It's recommended to use $CFG->debugdeveloper instead of debugging(..., DEBUG_DEVELOPER) to check for DEBUG_DEVELOPER level.
• Errors:
  • It's forbidden to set $CFG->debug in PHP code (but some exceptions in installers and friends). Instead set_debugging() must be used.
  • Never set $CFG->debugdeveloper in code.

It would be nice to have these covered by the code checker.