What does it mean when someone says, PHP isn’t scaleable?

There are many debates found on sites like Live Journal, and others. Where the major argument against PHP is that it isn’t scaleable, and that Perl is.
Comments to the effect that PHP should only be used to pull information from a database and that it’s amateurish and only for a novice programmer.

Also where does Java fit into this mix, for large “scaleable” web sites.