[mappu]

>This means that global variables, dynamic variables (variable-variables, variable function calls, etc) and references are not allowed.

Globals, sure, bad practice. Ideally we wouldn't have any. Dynamic variables make sense to exclude, it turns static analysis into the halting problem. Luckily they're a bad practice too since 5.3 introduced closures.

But references?

EDIT: It's by Anthony Ferrara! and some other pretty big names in the PHP community