Finally had time to edit the snippets, but doing so caused problems.
For the record, I went into /Applications/Sublime Text.app/Contents/MacOS/Packages/, renamed the PHP package with a .zip extension, unpacked it, opened/edited the snippet files, then re-zipped the directory (using OS-X right-click->Compress), and renamed the zipfile, removing the .zip extension.
When I relaunched ST3, I got a ton of errors saying the language file could not be found, and the ApplySyntax package stopped working. Retried the procedure several times, and it never clicked.
Is there anything special I need to do re zip parameters, permissions, etc.? Seems like I must be missing a step.