That is an anti-pattern, since it suffers from shell meta-character injection (and argument injection). Since PHP has mkdir, you should use that instead. And pcntl_exec() is the correct API for running processes.

Typical users of this likely don't care the slightest about whether anyone considers it an anti-pattern, because you use those in order to write utility scripts. And those who care would use tooling to detect issues like that anyways.