Rule "Using undefined variables"

<?php

class TestUnknownVariables
{
    public function badFunction($a)
    {
        $p = $a + 2;
        $p = $p - $pp; // Usage of undefined variable

        echo $p;
    }
    
    public function goodFunction($a)
    {
        global $pp;
        $p = $a + 2;
        $p = $p - $pp;

        echo $p;
    }
}