php获取当前月份的前（后）几个月

//获取当前月份的前一月
function GetMonth($sign)
{
    //得到系统的年月
    $tmp_date=date("Ym");
    //切割出年份
    $tmp_year=substr($tmp_date,0,4);
    //切割出月份
    $tmp_mon =substr($tmp_date,4,2);
    // 得到当前月份的下几月
    $tmp_nextmonth=mktime(0,0,0,$tmp_mon+$sign,1,$tmp_year);
    // 得到当前月份的前几月
    $tmp_forwardmonth=mktime(0,0,0,$tmp_mon-$sign,1,$tmp_year);
        return $fm_next_month=date("Ym",$tmp_forwardmonth);
    }