I'm using MAMP Pro and trying to get an old project built in PHP4 up and running. The project depends on register_globals to be turned on. I tried to update the php.ini file located in MAMP/conf/php4/php.ini, but nothing would change when I looked at the phpinfo() output.
After banging my head against the wall for awhile I turned to the forums at mamp.info. There is an importance difference between MAMP and MAMP PRO. If you want to update any of the ini file settings while using MAMP PRO goto File => Edit Template and select the ini you want to update.
The full forum post can be found here.