Obviously the PHP online manual is a “go-to” resource for any PHP dev.
Though this kind of reference site (like MDN too), while very useful for concise information and reference, can be hard going for learning. Sometimes you want something more digestible.
CSS Tricks is a favourite for all things CSS related.
That’s a good PDO reference. The author used to frequent the forum, but has not been seen for a while.
I can’t emphasise this enough, I see a lot of questions here (mostly in PHP) from beginners “learning” from Youtube’s self-proclaimed experts, posting some horrendous code. Of course they don’t know enough to know just how how bad it is, that is both student and teacher. It is the blind leading the blind, down a very dark path…
Then you have a hard time convincing them to un-learn everything they have already spent their time learning.