As PHP is becoming a widely used web server technology, more and more developers are comparing PHP with other technologies like J2EE or .NET.
It has been the butt of criticism as considered not scalable: totally wrong !
A system is scalable when is able to keep the performances under an increased load, mainly due to more users. Speaking about scripting execution velocity (that is a constant in that evaluation) is a completely different kettle of fish.
So, PHP, Python and Perl absolutely scale as J2EE/.NET if the server is upgraded or more parallel servers are added.
Note that all the session-related problems (sticky sessions etc..) are managed in a lower layer, so PHP has no problem with that.
An interesting book about scalability is :Building Scalable Web Sites – written by Cal Henderson, the chief software architect of Flickr (built with PHP)