MacOSX 10.8 (or maybe other recent ones) already has apache2. You can run it on terminal.

$ sudo apachectl start # or restart

If you enable PHP5, you comment off two lines in /private/etc/apache2/httpd.conf. They were at 117 and 118 line on my machine.

#LoadModule php5_module libexec/apache2/libphp5.so
#LoadModule hfs_apple_module libexec/apache2/mod_hfs_apple.so

To test if it works, create a simple .php one.

$ sudo touch /Library/WebServer/Document/hello.php
$ sudo chmod 777 /Library/WebServer/Document/hello.php
$ echo '<?php echo "Hello World"?>' /Library/WebServer/Document/hello.php

After then, please restart apache2 and access to http://localhost/hello.php.

It will work properly!