I've always been a fan of PHP too; it was my first programming language that I used to build a shipping application, but saying there are 'no unknown unknowns' in a language as quirky and old as PHP is akin to denying the sky is blue.
PHP 4 + 5 had so many weird quirks, and they lasted a long time because PHP 7 took so long to ship (note there was no PHP 6). When 7 hit, they followed it up with many minor releases to try deprecating/removing/correcting some of those quirks, which itself introduced more unknowns when refactoring legacy code. Suddenly the quirks you were used to for so long were now gone or behaving differently (or more sensibly at least), but this required even more refactoring to account for.
When a language is always changing or evolving you can never know in advance with certainty how new features or changes will change how existing code will work.