Hadoop application development, and PHP
Basically Hadoop lets you run you send your own java classes into the cluster to be run on the appropriate servers at the appropriate times. All non JVM languages API for Hadoop basically involve running pre-existing java classes, so nothing fancy is possible.
Also, hadoop is not really interactive framework. It is geared more for batch processing huge amount of data in somewhat reasonable time. The rule of a thumb is if you sub-job processes less then 15MB in one step you shouldn't bother with hadoop. The overhead of setting up the job and shuffling data and results around is just going to kill you.
There is nothing stopping you from creating your own hadoop API for PHP similarly how it is done for Python. However, PHP is not really popular with the crowd who does large scale data processing.