Hi!
I don't think anybody on this forum remembers all the built-in functions, I have big doubts whether any of us remember just the PHP3 functions' syntax, even from the most common 'String functions' or 'Array functions' section. Sometimes I forget the order of parameters to some pretty common ones. Just keep the PHP manual on the desktop or in the panel next to the Start button. Then search for a function, usually by typing the section name they are listed under.
The most important thing is to understand how everything works and you'll be able to program without problems with this knowledge.
Best regards,
Stas