As someone with experience in multiple languages but not necessarily great at either I discovered my weakness has been understanding design patterns and fundamental concepts. Some people could justifiably suggest that learning another language can inform the big picture for you (big picture relating to programming fundamentals). However other people might suggest that it's fine to stick with PHP and focus on design patterns and broader theory there.
However if your question is about job security, PHP is here to stay although I can't comment on the salary level for PHP devs in the future.