计算两个日期之间的星期日

I want to calculate all Sunday's between given two dates. I tried following code. It works fine if days are less but if i enter more days. It keeps processing and Maximum execution time exceeds i changed the time but it even keeps processing even execution time is 200sec.

代码是

<?php
$one="2013-01-01";
$two="2013-02-30";

$no=0;
for($i=$one;$i<=$two;$i++)
{

    $day=date("N",strtotime($i));
    if($day==7)
    {
    $no++;
    }
}
echo $no;

?>

请帮忙.

推荐答案

John Conde 的回答是正确的，但这里有一个更有效和更数学的解决方案:

$start = new DateTime('2013-01-06');
$end = new DateTime('2013-01-20');
$days = $start->diff($end, true)->days;

$sundays = intval($days / 7) + ($start->format('N') + $days % 7 >= 7);

echo $sundays;

$start = new DateTime('2013-01-06');
$end = new DateTime('2013-01-20');

$days = $start->diff($end, true)->days;

$sundays = intval($days / 7) + ($start->format('N') + $days % 7 >= 7);