Correctly naming things is one of the harder challenges in computer programming. Putting effort into naming intermediates that you're going to throw out is a waste. Plus, the more variables you introduce, the more likely you'll accidentally re-use a variable name somewhere down the line.

With PHP allowing variable initialization in one branch but not the other, and continuing execution by default when an undeclared variable is passed, declaring more variables can lead to an annoying class of bugs that would require significant (breaking) changes to the core language to completely eliminate.