YES to whether it's relevant. Regardless of opinions about PHP as a language, it's way too widely utilized to be irrelevant in the near future.
Whether it's worth investing time to be a PHP master is subjective. Personally, I won't be excited about being a crackerjack PHP dev, but if you find certain aspects of working with PHP compelling and suited to whatever you're building, it may be worthwhile. There's also plenty of work out there for people with PHP skills, so it could be pretty relevant with regard to employment.
Whether it's worth investing time to be a PHP master is subjective. Personally, I won't be excited about being a crackerjack PHP dev, but if you find certain aspects of working with PHP compelling and suited to whatever you're building, it may be worthwhile. There's also plenty of work out there for people with PHP skills, so it could be pretty relevant with regard to employment.