If you are running CGI PHP and are receiving the following errors or similar, you must check the "Xdebug" check box on your PHP tab.
[error] [client ::1] FastCGI: incomplete headers (0 bytes) received from server “/Applications/MAMP/fcgi-bin/php5.6.30.fcgi”
FastCGI: comm with server “/Applications/MAMP/fcgi-bin/php7.0.15.fcgi” aborted: idle timeout (30 sec)