public static function Ry($theta, array &$r) {
    $s;
    $c;
    $a00;
    $a01;
    $a02;
    $a20;
    $a21;
    $a22;

    $s = sin($theta);
    $c = cos($theta);

    $a00 = $c * $r[0][0] - $s * $r[2][0];
    $a01 = $c * $r[0][1] - $s * $r[2][1];
    $a02 = $c * $r[0][2] - $s * $r[2][2];
    $a20 = $s * $r[0][0] + $c * $r[2][0];
    $a21 = $s * $r[0][1] + $c * $r[2][1];
    $a22 = $s * $r[0][2] + $c * $r[2][2];

    $r[0][0] = $a00;
    $r[0][1] = $a01;
    $r[0][2] = $a02;
    $r[2][0] = $a20;
    $r[2][1] = $a21;
    $r[2][2] = $a22;

    return;
  }