自定义时间函数

<?php
/**
 * @desc获取指定的日期
 * @ string $vdate 2000-01-31
 * @parma int $vnum +~-1
 * @param string $vtype  day/month/year
 * @retuen string date
 */
function datecount($vdate,$vnum,$vtype){
    $day = date('j',strtotime($vdate));
    $month = date('n',strtotime($vdate));
    $year = date('Y',strtotime($vdate));
    switch($vtype){
        case 'day':
           if($vnum >= 0){
                $day = $day + abs($vnum);
            }else{
                $day = $day - abs($vnum);
            }
        break;
        case 'month':
           if($vnum >= 0){
				$month = $month + abs($vnum);
            }else{
                $month = $month - abs($vnum);
            }
        break;
        case 'year':
           if($vnum >= 0){
                $year = $year + abs($vnum);
            }else{
                $year = $year - abs($vnum);
            }
        break;
        default :
            break;
    }
    $time = mktime(0,0,0,$month,$day,$year);
    return date('Y-m-d',$time);
}

echo  datecount('2000-01-31',-1,'month');