You’ve started to use OS X Yosemite and even if you do use the official release and not the beta there is some trouble starting Apache with MAMP? The problem do occur using both MAMP (I’m using version 126.96.36.199) and MAMP PRO.
There is a simple solution to the problem:
/Applications/MAMP/bin/apache2/bin using Finder (the ”bin” folder will be a shortcut), and rename the file
_envvars - add a underscore to the filename only.
Thats it, now start the servers in MAMP or MAMP PRO and you´re good to go.