[EDIT] The last version of MAMP with the last version of OSX Yosemite works fine.
I installed the beta of Yosemite two month ago, no problem with MAMP. With the last
I've found this video. This works for me.
Go to MAMP in Applications
then, bin > apache2 > bin and rename 'envvars' to '_envvars'
https://www.youtube.com/watch?v=brpKAM2WXhk