计算两个日期之间周末天数in php

public function getWeekendDays($fromDate,$toDate)
{
$start_z = date('z',strtotime($fromDate));
$end_z = date('z',strtotime($toDate));
$weeksOffset = ($end_z/7) - ($start_z/7);
$start_N = date('N',  strtotime($fromDate));
$end_N = date('N',strtotime($toDate));
if(($start_N+$end_N)>10)
{
if($start_N>5)
$weeksOffset +=(5-$start_N)/2;
if($end_N>5)
$weeksOffset +=($end_N-5)/2;
}
return $weeksOffset*2;
}