Realistically, the best way to learn a language is to make a small project that does something useful. But if you're still wanting to do challenges rather than a project, you're just in time to participate in Advent of Code 2017 [0]. The only 'gamified' part is the leaderboard which is based on how long it takes you to generate a correct answer since the challenge became available. Another challenge that, while not generally suited for a PHP programmer but still possible, is the Synacor Challenge [1] , which involves creating a virtual machine and running a provided binary in that virtual machine to find codes to submit.

[0] https://adventofcode.com/ [1] https://challenge.synacor.com/