为什么提交跑到空白页面？

<form>

<input type="text" name="year" placeholder="请输入年份">

<input type="submit" name="计算">

</form>

<?php 

$year=$_GET['year'];

if($year%4==0&&$year%100==0||$year%400==0)

{

echo "今年是闰年";

}

else{

echo "今年是平年";

}

 ?>