[TYPO3] tutorials on writing extensions? How do you make php work in typo3?

[dave typo]

Is there some tutorials online to show how to properly write extensions,
using  global variables and tslib and conf and everything else? I'm having a
difficult time following the code that is prexisiting in various extensions,
and I have generally no idea which variables to reference within the typo3
core.


any place I could get a good start?

where did any of you get the start to write extensions?


thanks,

-dave