The PornHub.com site (definitely NSFW) is a high profile site that, as it turns out, uses PHP for a lot of its functionality. In this interesting article from the Evondie Security Research Group they show how they "broke PHP and hacked PornHub (and earned a $20k USD bug bounty in the process). Don't worry, the article itself is "safe for work" as it's only descriptions and code examples of how the hack was performed.
Pornhub’s bug bounty program and its relatively high rewards on Hackerone caught our attention. That’s why we have taken the perspective of an advanced attacker with the full intent to get as deep as possible into the system, focusing on one main goal: gaining remote code execution capabilities. Thus, we left no stone unturned and attacked what Pornhub is built upon: PHP.
The post then walks you, step-by-step, through the process they followed to discover the exploit. The main entry point was through PornHub's use of the unserialize function that included a flaw allowing for code execution when a specially crafted object was injected. With the help of this they were able to "leak" out of the PHP execution and inject custom C code to be executed in the local environment. This was, in turn, then used to execute a file_get_contents on the local
/etc/password file and return its contents.