C++ has been around for a long time, which means there are lots of resources (many of them free) you could use to learn it. However, it has changed a great deal over the decades. You can see that in the answers here - people calling it insane and difficult are generally referring to features inherited from C or features that modern C++ developers rarely use, like manual memory management. You would probably find modern C++ quite easy to learn even with only PHP and CSS background. However, that won't help you if quants don't use modern C++.
I just published a course called C++ Fundamentals and since Pluralsight offers a free trial, you could watch the first few hours of it free to see whether learning C++ is something you can do or not. Then you could either go and buy a book or sign up for a paid course to carry yourself further down the path. Be very careful with the book though. If the first chapter is full of char*, int, while loops that increment pointers, and functions whose names start str, don't buy that book. Look for the first chapter using cout, string, and vector. Look for coverage of lambdas and auto, and look for a tone of simplicity and control. You can find a TON of books, blogs, web pages, FAQs, courses, and so on that teach the hard insane C++ people love to tell stories about. I don't recommend you learn that language first.
If it turns out the quants don't use modern C++, I suppose you could learn the old stuff once you were comfortable with the new. It would be hard. You would also have to learn some specific libraries that do financial calculations, and probably something for multicore and manycore work. For example as a Visual C++ user I would learn PPL and C++ AMP. But start with a foundation of knowing the syntax of the language and the philosophy of those who use it.