From a strictly newbie point of view starting to learn both Perl and PHP one of the things that immediately struck me with PHP was the built in functionality that allowed database manipulation and subsequent display in web pages so readily.
This gives a learner instant gratification that they have a working solution/answer and I think that initial buzz sticks, they then continue to use it regardless whether it is the best option or not.
Again from experience of “taught languages” as opposed to “self taught” it is unusual for a University student in the UK to get the opportunity to learn both. It is far more likely that PHP will be an option Perl being offered only on very few courses Artificial Intelligence and perhaps Information Systems for example.