Many IDEs are now smart enough to read your code and understand what you are doing, which means they can prompt you with help. Forgotten the exact ordering of parameters for mysql_connect() ? That is fine, because an IDE with code insight will prompt you for them, even telling you which parameter you are currently entering. Do not remember what functions object $foo has? Again, not a problem - code insight will pop up a list of all the functions and variables of each object, and you can just click the one you want.
Having an editor with code insight built in means you spend much less time looking at the manual, or looking at your own files remembering what values a function returns, and the end result is that you spend more time actually programming new features.