检查日期是否大于字符串

Question

我有一个从API调用中获取的数组。数组如下所示：

  [3] => Array
        (
            [project_id] => project:eead2382e9d679814c13c77ab857ae53
            [start_date] => 2017-01-16
            [end_date] => 2019-11-15
        )

    [4] => Array
        (
            [project_id] => project:096b2fc2bc5133784c13c77ab857ae53
            [start_date] => 2017-03-08
            [end_date] => 2017-06-30
        )

我正在遍历所有结果，并检查end_date值是否等于今天或大于今天。像这样：

$today = date("Y-m-d");

$qualified = [];

foreach($results as $result) {
    if($result['end_date'] > $today){
        array_push($qualified, $result);
    }
}

echo '<pre>';
print_r($qualified);
echo $today;

但是我得到一个空数组eventho，它的值比今天大。难道我做错了什么？