[Update - see here. I wouldn't recommend the Apple PHP distro anymore as it has shortcomings. Roll your MAMP!]
See full description for PHP here, for activating SSI here.

Add PHP with debugging

Activate: goto /private/etc/apache2/httpd.conf and uncomment LoadModule php5_module . Save.
Add debugging: In Terminal type:
cd /private/etc
sudo cp php.ini.default php.ini
Edit said php.ini: change
error_reporting = E_ALL & ~E_NOTICE to
error_reporting = E_ALL .Save

Add SSI

Go back to /private/etc/apache2/httpd.conf…

1. Uncomment these lines from the <ifmodule mime_module> stanza:
AddType text/html .shtml and
AddOutputFilter INCLUDES .shtml

2. Add “Includes” to the “Options” statement in the <directory "/Library/WebServer/Documents"> stanza (so it now reads
Options Indexes FollowSymLinks MultiViews Includes)

3. [Re]start Apache with sudo apachectl graceful

Job’s a good ‘un.