Duplicated code in projects is a frequent thing and also the one ripe for factoring out in a new class or function. Cut/Paste coding is a common development practice among programmers, a lot of which can lead to code size increase and maintenance nightmares. PHPCPD (php copy paste detector) is a PEAR tool that makes it easier to detect duplicate code in php projects. Below is a short tutorial on the PHPCPD package.
You can either install the tool via a PEAR channel or directly from the github site. Once its downloaded and extracted you can immediately run it on the subdirectory of your choice. He also includes a more extended example - a search on a minimum of 5 lines of 70 tokens found to trip the filter in finding cloned functionality.