php 获取指定月份的天数

<?php
$days = cal_days_in_month(CAL_GREGORIAN, 8, 2011);
echo "返回2011-4的天数 ".$days."<br/>";
$days = date('t', strtotime("2011-8-1"));
echo "返回2011-4的天数 ".$days."<br/>";
$days = date("t");
echo "当前月的天数 ".$days."<br/>";
?>