[TYPO3-dev] Quick PHP question

[Oliver Salzburg]

I just read the following part in the TYPO3 core API documentation:

> Hooks are the recommended way of extending TYPO3 compared to
> extending the PHP classes with a child class (see "XCLASS
> extensions"). It is so because only one extension of a PHP class can
> exist at a time while hooks may allow many different user-designed
> processing functions to occur.

The statement "only one extension of a PHP class can exist at a time"
is really confusing me.
Was that true at some point in time or did I just skip some important
pages in my PHP lecture?