Leetcode 13: Roman to Integers

Here is my solution to this problem in PHP:

    private $v = ['M' = > 1000, 'D' = > 500, 'C' = > 100, 'L' = > 50, 'X' = > 10, 'V' = > 5, 'I' = > 1];
  /**
    * @param String $s
    * @return Integer
    */
    function romanToInt($s) {
        $n = 0;
        $last = '';
        for ($i=0;$i < strlen  ($s  );   $i++)   {
             $n   +=   $this-  >v[$s[$i]];
            if (($s[$i] == 'V' || $s[$i] == 'X')  &&  $last == 'I') {
                $n = $n - 2;
            } elseif (($s[$i] == 'L' || $s[$i] == 'C')  &&  $last == 'X') {
                $n -= 20;
            } elseif (($s[$i] == 'D' || $s[$i] == 'M')  &&  $last == 'C') {
                $n -= 200;
            }
            $last = $s[$i];
        }

        return $n;
    }
}