Like many people, many years ago I learned php. It was easy to deploy, and make simple dynamic websites. Many hosting services provided support for it. I was quite happy to use it for these things.
Then in 2008 I interned at Facebook. At the time, there had not yet been any attempts to improve the language by Facebook itself. Using php in an actual codebase - real world software - made me realize just how terrible it was. I vowed to never use it again. I even presented some of the worst aspects of php in my intern presentation to the executive team. Mark was quite intrigued. At the time there were talks of how to deal with many of the problems, but nothing had been figured out by time I left.
Its deficiencies are too numerous to simply list here, so I'll give you a high level summary (which many have also said similar things). Php is a giant hacked language originally not designed for real software and then features added on. It's syntax is the worst of any used language; it's language features are severely lacking; its implementations are contradictory, confusing, and destructive. In short, it is not easy to read, it is easy to shoot yourself in the foot, it is not able to scale both in complexity and performance; and its libraries are a mess.