public static function getHeader($key)
    {
        if ( ! array_key_exists($key, HttpCache::$header)) {
            return NULL;
        }

        return HttpCache::$header[$key];
    }
public static function getHeader($key)
    {
        if ( ! array_key_exists($key, HttpCache::$header)) {
            return NULL;
        }

        return HttpCache::$header[$key];
    }
public static function getHeader ( $ key ) { if ( ! array_key_exists ( $ key , HttpCache :: $ header ) ) { return NULL ; } return HttpCache :: $ header [ $ key ] ; }
get http response header.

@param  string  $key
@return string
get http response header .