Is there a function to write a PHP array directly to a cell range and to read a cell range directly into PHP array?
Yes and yes.
The worksheet fromArray() and rangeToArray() methods.
I am also missing a documentation of all functions. I found a bunch of html files in the API folder but clicking on the links reported errors.
I also found some Word documents but they seem to get you started but do not contain all available functions.
Access the API docs through a web server http request, not simply pointing the browser at the files.