//$startdate是开始时间，$enddate是结束时间
<?php
$startdate="2011-3-15 11:50:00";

$enddate="2012-12-12 12:12:12";

$date=floor((strtotime($enddate)-strtotime($startdate))/86400);
echo "相差天数：".$date."天<br/><br/>";

$hour=floor((strtotime($enddate)-strtotime($startdate))%86400/3600);
echo "相差小时数：".$hour."小时<br/><br/>";

$minute=floor((strtotime($enddate)-strtotime($startdate))%86400/60);
echo "相差分钟数：".$minute."分钟<br/><br/>";

$second=floor((strtotime($enddate)-strtotime($startdate))%86400%60);
echo "相差秒数：".$second."秒";
?>