Let’s understand this way. Around 70-80% of the website in the world is running on PHP/MySql. Also, it’s a very stable programing language so; it’s not going to die-down so early.
But, since it’s based on traditional technology so MEAN stack is fast overtaking traditional LAMP.
So, my suggestion is that, you should learn PHP/MySql to at least at fundamental level so that you know the fundamental of web programing and how the web programing works.
Another advantage is that most of the existing we sites are in PHP/MySql so, understanding API building and fetching will be helpful for you. You will better understand how these work.