On the CodeUtopia blog Jani Hartikainen has posted some thoughts on generic collections in PHP and a class he's created to try to introduce them to the language.

Strictly typed languages usually use "generic" collection classes instead of arrays. They are kind of like PHP arrays which the programmer can tell which type of items to accept. This is of course only natural when you don't have dynamic typing, but it can also be useful for avoiding programming errors, so I thought I'd try making a basic generic collection class in PHP...

He shows how ti works with a simple code example - creating a new collection type (a string) and pushing the data into it. Calling the add() method on the string throws an exception because of the data type defined. You can grab the code from his svn repository.