Graph::GetKey PHP Method

    protected function GetKey($index)
    {
        $k = array_keys($this->values[0]);
        // this works around a strange bug - if you just return the key at $index,
        // for a non-associative array it repeats some!
        if (!$this->force_assoc && is_int($k[0])) {
            return $index;
        }
        if (isset($k[$index])) {
            $index = (string) $index;
            $index = (int) $index;
            return $k[$index];
        }
        return NULL;
    }