It's mostly things like parameter order for certain built-ins that aren't intuitive and/or finding built-ins I've never used or heard of because there are so many obscure ones (there are nearly six thousand) that keep me coming back. For example, how do you disable errors in a DOMDocument? It ought to be as easy as DOMDocument->setAttribute("errors", "false"), but no it's a separate function call to libxml_use_internal_errors(true). There are like five other similar function calls for this too. PHP has abstraction leaks and little inconsistencies like this everywhere.
Granted, languages with small standard libraries, like JavaScript, by contrast get an easy cop out to blame the community of various package maintainers here. But at least in that case someone can always just write a better library.
Much more frequently I’m looking up something related to the tools and libraries I’m using.
The only surprising thing I’ve learned about PHP at the language level in the last 6-10 months is that you can call array_filter() without a callback to cull nulls.