php判断平年闰年

公历闰年判定遵循的规律为：四年一闰，百年不闰，四百年再闰。

$year=1900;
if(($year%4 == 0 && $year%100 != 0) || ($year%400 == 0 )) {
echo "$year is run yerar. 
";
}else {

echo "$year is ping year. 
";
}

$a=10;
$b=$a++ + ++$a;
echo $a."
";
echo $b."
";
?>

输出:
10
22

原因:
左自增比右自增优先级高,所以是$b=$a + (++(++$a))=10+12=22