In order to get any sort of AI and / or learning system in, we need: * external libraries, PHP is Just Not Ready for this. (performance, memory, and library wise) Depends, php is perfectly feasible for a lot of AI tasks - look at the spam.module. It is a classifier. It performs reasonably well.
It is a challenge as well. Depends. From some of the evaluations I've done some time ago, it is perfectly reasonable to expect a php system to be used for user profiling and generating adaptive websites. Learning algorithms are not nessesarily heavy.
Hmm. If not PHP, then were is the logic? I'm most familiar with the Weka toolkit: http://www.cs.waikato.ac.nz/ml/weka/
Have a look at triana http://www.trianacode.org , you can get a web services environment wrapping the weka classes with that. And the guys are experimenting a lot with it. But it is heavy. It is not for Joe X. There is orange as well: http://www.ailab.si/orange And I agree with your logic. There is place for such things in a system like Drupal. That is power on your fingertips.