PHP日期处理函数

<?php
/*
* author wuhan syspro zhangchao
* date 2010/12/23
*/

// next day
function next_day($theday){
if(!$theday){
$theday=date(DATE_W3C);
}
return next_days($theday,1);
}

// pre day
function pre_day($theday){
if(!$theday){
$theday=date(DATE_W3C);
}
return next_days($theday,-1);
}

// next days
function next_days($theday,$incr){
if(!$theday){
$theday=date(DATE_W3C);
}
if(!$incr){
$incr=1;
}
$nextday=strtotime($theday)+24*60*60*$incr;
$nextday=date(DATE_W3C,$nextday);
return $nextday;
}

// days between two date
function interval_days($firstday,$lastday){
if(!$firstday){
$firstday=date(DATE_W3C);
}
if(!$lastday){
$lastday=date(DATE_W3C);
}
$firstday=strtotime($firstday);
$lastday=strtotime($lastday);
$days=($lastday-$firstday)/60/60/24;
return $days;
}

// first day
function first_day($theday){
if(!$theday){
$theday=date(DATE_W3C);
}
$firstday = date('Y-m-01', strtotime($theday));
return $firstday;
}

// last day
function last_day($theday){
if(!$theday){
$theday=date(DATE_W3C);
}
$firstday=first_day($theday);
$lastday = date('Y-m-d', strtotime("$firstday +1 month -1 day"));
return $lastday;
}

?>