uninitialised variables and E_NOTICE

 missed that an E_NOTICE is emitted if an uninitialised variable is accessed
"If a variable is used which was not defined so far then PHP uses `NULL` instead" sounds a bit wishy-washy, "used" is ambiguous.

Maybe something like "If a variable that is not defined is referenced in an expression, then the reference to the variable evaluates to `NULL` and an `E_NOTICE` is emitted"? Though "referenced" might be confused with by-reference assignment/calling.