Requirements

Requirements --  Dependencies, time and memory settings

PHP-Extensions

Both extensions should be avaible in common PHP distributions.

Execution time

I.e. PHPDoc needs between 3 and 10 min for creating an API documentation of the whole PEAR project depending on your computers performance.

Memory limit

It could be necessary to increase the memory limit. Especially when you are using Windows and PHP crashes while runing PHPDoc.
memory_limit = 67108846		; Maximum amount of memory a script may consume (64MB)