I wouldn't put PHP close to Flask in the same sentence. Even though PHP has quite an extensive function library and is a template language in itself, it doesn't provide enough to compare even to a micro framework like Flask.
To come close you would probably end up using a framework written in PHP. At that point the language doesn't matter all that much.
I'm helping a friend writing a simple wiki-like application and due to him having to explain every part of the program we decided to write as much ourselves as possible and only use external libraries when absolutely necessary. Me trying him to explain what Django does and why it would help him didn't end well.
I'm not the absolute master of Python web frameworks, but I know enough to come by. But I tell you, writing clean, understandable, side-effect free and secure code in PHP is much harder in comparison than I expected.
There are thousands of tutorials on the web. But a lot of them show things done wrong, deprecated or blatantly insecure. The language is quite a mess and you can do the same thing 10 different ways, 8 of which are deprecated and maybe 1 that works with unicode. Not that you would know it's deprecated, the original mysql API is still not officially called deprecated, but it sucks big time.
I reckon knowing how to use Flask and Django makes it harder for me to write code the hard way. I'm replicating a lot of functionalty in PHP. But at the same time I know that I'm not good enough to do so.
And doing so costs time anyway. I would have probably finished his application in a quarter of the time using Django, heck, even Flask. And it might even hold up to industry standards. My PHP code not so much. Somebody would probably end up dead.