I updated to the latest version of yosemite last night.

You apparently have to go to your MAMP folder in Applications. Go to bin -> apache2 -> bin.

Then rename the file from envvars to _envvars.

I also came across another problem of it not working correctly. Make sure the ports are Apache defaults, NOT MAMP defaults (port 80 etc). If it works straight away, this should not apply to you.