Several sites on the web gave quick guides for this, but still, as always there are a few things you need to keep in mind for it all to work. I will try to walk through the process as detailed as possible:
First start by installing Xcode. You can get the latest version from the App Store. NOTE that the App Store only have the latest version of Xcode, so if you need a version for Snow Leopard or older, you need to dig deep on the web (https://connect.apple.com is a good place to start).
Next you need to install the Command Line Tools from Xcode. You do this by opening Xcode, go to Preferences (CMD + ,) -> Downloads -> Components and hit install on the Command Line Tools. You can get the Command Line Tools without installing Xcode via https://connect.apple.com
Now you need to find out what version of PHP is installed on OSX
$ php -v
PHP 5.3.10 with Suhosin-Patch (cli) (built: Feb 20 2012 22:55:53) Copyright (c) 1997-2012 The PHP Group Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
and then get the source code for that version
$ curl -O http://us.php.net/distributions/php-5.3.10.tar.gz
When the download have finished, you should unpack, find the PCNTL extension, phpize, configure, make and then make install
$ tar -xzvf php-5.3.10.tar.gf
$ cd php-5.3.10/ext/pcntl
$ sudo make install
If phpize is complaining, you are probably missing autoconf. The easiest way to install autoconf is via Homebrew. You install Homebrew by doing
$ /usr/bin/ruby -e "$(/usr/bin/curl -fsSL https://raw.github.com/mxcl/homebrew/master/Library/Contributions/install_homebrew.rb)"
The first thing you need to do afterwards is to run
$ brew doctor
to find out if any dependencies are missing. Homebrew will telle you what to do, if any is missing.
Lastly you need to enable the PCNTL extension. Do this by adding the following to your php.ini
if you do not know where your php.ini file are, you probably need to create on, by doing
sudo cp /private/etc/php.ini.default /private/etc/php.ini
Restart Apache and verify that the PCNTL extension are loaded correctly
$ sudo apachectl restart
$ php -m | grep pcntl
If the PCNTL extension for some reason is not loaded, try and copy the pcntl.so from the compiled source code to the extensions directory
$ sudo cp php-5.3.10/ext/pcntl/modules/pcntl.so /usr/lib/php/extensions/no-debug-non-zts-10090626/
Restart apache and verify the the PCNTL extension is loaded.