Linking homebrew, php, mcrypt, and localhost:8000
I'm not familiar with OpenCart but you should check you path and make sure the homebrew php is the default.   

In the terminal: 

$ which php
/usr/bin/php
$ php -v
PHP 5.5.14 (cli) (built: Sep  9 2014 19:09:25) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2014 Zend Technologies

$ which mongod 
/usr/local/bin/mongod
$ ls -al /usr/local/bin/mongod
lrwxr-xr-x  1  ----  admin  34 Dec  8 17:25 /usr/local/bin/mongod -&gt; ../Cellar/mongodb/2.6.5/bin/mongod


In this example I've installed mongod from homebrew.  I've never installed php but it looks like it exists on my system by default.  I'm guessing your system is using the standard php and not the brew one.  

Try running your server like this:

/usr/local/bin/php -S localhost:8000