Commas are used in PHP as a sequence of things, not multiple options. This single-pipe notation is already established as the further-up comments states.
> Commas are used in PHP as a sequence of things
Choosing a pipe because it's easier for an obscure language feature, is not least astonishment. Commas are used for lists. The pipe is a bad choice among a nice feature.
Commas would require changing the syntax and implementation of catch. This way exception names just become first class subjects that can be combined with an overloaded operator.