Apache is logging PHP Fatal error: Call to undefined function json_decode(). After some googling, it seems this problem is a result of not having the latest version of php. Oddly, running php --version ouputs

PHP 5.5.1-2+debphp.org~precise+2 (cli) (built: Aug  6 2013 10:49:43) 
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2013 Zend Technologies
    with Zend OPcache v7.0.2-dev, Copyright (c) 1999-2013, by Zend Technologies
    with Xdebug v2.2.3, Copyright (c) 2002-2013, by Derick Rethans

Clearly showing that my php version is greater than 5.1.

Any ideas as to what could cause this, or the steps to take to find out?

EDIT: The result of a script echoing phpversion is 5.5.1-2+debphp.org~precise

EDIT: The result of phpinfo() is also PHP Version 5.5.1-2+debphp.org~precise+2

Furthermore, in the phpinfo(), it lists the module JSON and the module authors (Omar Kilani, Scott MacVicar).