How do you change php.ini values in MAMP Pro?
It turns out that browsing to the files and editing them directly is not the best way, you should use MAMP's built-in Template Editor.
You can turn this on by going to:
File > Edit Template > PHP > The PHP version you're running
If you can't see the File menu at all, it could be that you have the Hide Dock Icon preference set. In this case you'll need to uncheck that preference, restart MAMP, then use the template editor to tweak my php.ini file, save, restart Apache, done!