Using the bundled PHP

PHP has come standard with Macs since OS X version 10.0.0. Enabling PHP with the default web server requires uncommenting a few lines in the Apache configuration file httpd.conf whereas the CGI and/or CLI are enabled by default (easily accessible via the Terminal program).

Enabling PHP using the instructions below is meant for quickly setting up a local development environment. It's highly recommended to always upgrade PHP to the newest version. Like most live software, newer versions are created to fix bugs and add features and PHP being is no different. See the appropriate MAC OS X installation documentation for further details. The following instructions are geared towards a beginner with details provided for getting a default setup to work. All users are encouraged to compile, or install a new packaged version.

The standard installation type is using mod_php, and enabling the bundled mod_php on Mac OS X for the Apache web server (the default web server, that is accessible via System Preferences) involves the following steps:

Locate and open the Apache configuration file. By default, the location is as follows: /etc/httpd/httpd.conf
Using Finder or Spotlight to find this file may prove difficult as by default it's private and owned by the root user.
Note: One way to open this is by using a Unix based text editor in the Terminal, for example nano, and because the file is owned by root we'll use the sudo command to open it (as root) so for example type the following into the Terminal Application (after, it will prompt for a password): sudo nano /etc/httpd/httpd.conf
Noteworthy nano commands: ^w (search), ^o (save), and ^x (exit) where ^ represents the Ctrl key.
With a text editor, uncomment the lines (by removing the #) that look similar to the following (these two lines are often not together, locate them both in the file):
# LoadModule php4_module libexec/httpd/libphp4.so # AddModule mod_php4.c
Notice the location/path. When building PHP in the future, the above files should be replaced or commented out.

Be sure the desired extensions will parse as PHP (examples: .php .html and .inc)

Due to the following statement already existing in httpd.conf (as of Mac Panther), once PHP is enabled the .php files will automatically parse as PHP.

<IfModule mod_php4.c>
    # If php is turned on, we respect .php and .phps files.
    AddType application/x-httpd-php .php
    AddType application/x-httpd-php-source .phps

    # Since most users will want index.php to work we
    # also automatically enable index.php
    <IfModule mod_dir.c>
        DirectoryIndex index.html index.php
    </IfModule>
</IfModule>

Be sure the DirectoryIndex loads the desired default index file
This is also set in httpd.conf. Typically index.php and index.html are used. By default index.php is enabled because it's also in the PHP check shown above. Adjust accordingly.
Set the php.ini location or use the default
A typical default location on Mac OS X is /usr/local/php/php.ini and a call to phpinfo() will reveal this information. If a php.ini is not used, PHP will use all default values. See also the related FAQ on finding php.ini.
Locate or set the DocumentRoot
This is the root directory for all the web files. Files in this directory are served from the web server so the PHP files will parse as PHP before outputting them to the browser. A typical default path is /Library/WebServer/Documents but this can be set to anything in httpd.conf. Alternatively, the default DocumentRoot for individual users is /Users/yourusername/Sites
Create a phpinfo() file
The phpinfo() function will display information about PHP. Consider creating a file in the DocumentRoot with the following PHP code:
<?php phpinfo(); ?>
Restart Apache, and load the PHP file created above
To restart, either execute sudo apachectl graceful in the shell or stop/start the "Personal Web Server" option in the OS X System Preferences. By default, loading local files in the browser will have an URL like so: http://localhost/info.php Or using the DocumentRoot in the user directory is another option and would end up looking like: http://localhost/~yourusername/info.php

The CLI (or CGI in older versions) is appropriately named php and likely exists as /usr/bin/php. Open up the terminal, read the command line section of the PHP manual, and execute php -v to check the PHP version of this PHP binary. A call to phpinfo() will also reveal this information.