As somebody who maintains and develops a commercial WordPress site as well as doing Rails work, my impression is the opposite: Rails projects tend to be moderately easy to follow while PHP projects tend to be composed of 10kloc balls of intertwined logic, state and IO with no comments aside from a healthy helping of:

    // $var = foo(bar);
    $var = baz(bar);

Perhaps we're both encountering a bit of luck rather than accurately assessing the two languages' ecosystems.