I read up on the ST3 documentation, and it says
Overriding Files From a Zipped Package
To override a file in an existing package, just create a file with the same name under the Packages/<Package Name> directory.
For example to override the file function.sublime-snippet in the Python.sublime-package package that ships with Sublime Text, create a directory called Python under the <data_path>/Packages directory, and place your function.sublime-snippet file there.
If I create an empty file ~/Library/Application Support/Sublime Text 3/Packages/PHP/define(-).sublime-snippet ST3 crashes upon start. If I copy the snippet from the ZIP file and change the tab trigger to something else, now both the trigger I don't like in the first place and the new trigger work.