PHP Fatal error: Call to undefined function json_decode()

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).