Moodle is a popular open source LMS (learning management system) written in PHP that could use a ton of improvement, especially on the front end. It's not necessarily bad, but it is very outdated compared to offerings by private companies.
DokuWiki can always use help. https://www.dokuwiki.org
Are there any specific libraries or tools written in PHP that you use in your day-to-day life? Perhaps having a look into their state could yield some interesting opportunities for you to contribute. That way you'd also get an extra incentive to keep contributing, as you'd personally get to enjoy the results, too.
What are your interests beyond "PHP developer"?
There's a lot of open source PHP projects. Type a random word relating to one of your interests into https://packagist.org and you'll likely find several.
My suggestion for what it's worth is to help the MantisBT project a bugtracker used by a lot projects.
pfSense could always use some help!
Pi-Hole :)
You can contribute to what 35% of the internet runs on: https://github.com/WordPress/WordPress
Contributing to Drupal modules or underlying (symfony and other) libraries is very fulfilling. And the conferences are really nice (and big) with lots of interesting people there with all sorts of problems. There is all kinds of difficult there if you like difficult.