试图在表单提交时插入空日期,但得到00:00

我试图在表单插入中允许空日期(对于永久类型禁止),但是如果字段留空,它将插入 0000-00-00 00:00:00

这个 BannedMembers 表如下所示:

CREATE TABLE BannedMembers
(
 BanID         INT NOT NULL AUTO_INCREMENT,
 MemberID      INT NOT NULL ,
 WarningSent   BIT NOT NULL ,
 BannedDate    DATETIME NOT NULL ,
 BanDuration   DATETIME NULL ,

 PRIMARY KEY  (BanID),
 FOREIGN KEY (MemberID)
    REFERENCES Members(MemberID)
);

<form method="post" name="banmember_form" id="banmember_form" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
    Banned Until? (Leave empty for permanent)
    <input type="date" class="form-control" name="banmember_duration" id="banmember_duration" placeholder="dd/mm/yyyy">
    <button type="submit" name="addbannedmember_submit" id="addbannedmember_submit">Add</button>
</form>

<?php
if(isset($_POST["addbannedmember_submit"]))
{
    //Check if the duration is empty (for permanent bans)
    if (empty($_POST["banmember_duration"]))
    {
        $banDuration = NULL ;
    }
    else
    {
        $banDuration = $_POST["banmember_duration"];
    }
    $addBannedMemberSql = "INSERT INTO BannedMembers(MemberID, BannedDate, 
    BanDuration)
    VALUES (".$_POST["banmember_name"]."',NOW(),'$banDuration') ";
    $conn->exec($addBannedMemberSql);
    echo "<meta http-equiv='refresh' content='0'>";
    }
?>

从我所读到的,以及我所能发现的,上面的空值被存储为 '' 所以当它插入时,它只是默认返回到 00:00...etc '' 当别人说要包括他们的时候,我试过了 ({$banDuration}) 在我的 INSERT