If you are working on PHP/MySql and you are thinking to learn MEAN Stack, it will be really a very good decision. If you learn MEAN Stack, you will then become a full-stack developer.
PHP/MySql is based on LAMP technology which has been here for long and it’s not going to lose its sheen in near future. But, it’s not good for real-time applications like chat-applications or multi-player games. PHP/MySql lags behind In this regard.
NodeJs is an evented-loop based server handling platform, which is used for developing applications which requires persistent connection and can handle millions of users together. And NodeJs fits perfectly in this regard.
So, if you are a PHP/MySql developer and you are thinking to learn MEAN Stack it will be a very good decision for your career.