我正在尝试插入我的sql数据库。因为某种原因我得到了 Incorrect integer value: '' for column 'user_id' at row 1 错误。
以下是我的表架构：

如你所见， user_id 是引用另一个表中的列的外键。我不确定你是否需要这些信息，但是是的。我从我的insert php中得到了错误。
以下是我的插入php：

<?php
require_once ('database.php');

if (isset($_POST['send'])) {

$user_id = $_POST['user_id'];
$full_name = $_POST['full_name'];
$phone = $_POST['phone'];
$email = $_POST['email'];
$fromdate = ($_POST['fromdate']);
$todate = ($_POST['todate']);
$reason = ($_POST['reason']);
$status = ($_POST['status']);

{
$database->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$insert_query = "INSERT INTO leaves (user_id, full_name, phone, email, fromdate, todate, reason, status)
VALUES (?, ?, ?, ?, ?, ?, ?, ?)";

$insert = $database->prepare($insert_query);
$insert->execute(array($user_id, $full_name, $phone, $email, $fromdate, $todate, $reason, $status));

echo "<script>alert('Successfully sent!'); window.location='leaveform.php'</script>";
}
}
?>

形式：

<div class="container">
    <div class="card">
        <div class="form__name" style="font-family: Questrial; ">
            Leave Application Form
        </div>
        <div class="time__container">
            <div class="section">
                <div class="box">
                    1
                </div><span>Date</span>
            </div>
            <form action="insert-leave.php" class="form__time" method="post">
                <div class="date">
                    <label for="date">From</label> <input id="date" type="date" name="fromdate" required>
                </div>
				&nbsp;
				&nbsp;
				&nbsp;
                <div class="date">
                    <label for="date">To</label> <input id="date" type="date" name="todate" required>
                </div>
                <div class="timezone">
                    <label for="timezone"></label> 
                        </select>
                </div>
        </div>
        <div class="message__container">
            <div class="section">
                <div class="box">
					2

                </div><span>Reason</span>
            </div>
            <textarea cols="50" rows="5" name="reason" required></textarea>
        </div>
        <div class="contact__container">
            <div class="section">
                <div class="box">
					3

                </div><span>Contact Information</span>
            </div>
            <div class="form__contact">
                <div class="cname">
                    <label for="cname">Name</label> <input name="full_name" type="text" readonly="readonly" value="<?php echo $fullname; ?>" required>
                </div>
                <div class="cnum">
                    <label for="cnum">Phone Number</label> <input id="cnum" type="text" readonly="readonly" name="phone" value="<?php echo $contact; ?>" required>
                </div>
                <div class="email">
                    <label for="cemail">Email</label> <input id="cemail" name="email" readonly="readonly" value="<?php echo $email; ?>" required>
                </div>
				<div class="email">
                    <label for="cemail"></label> <input type="hidden" name="status" required>
                </div>
				<div class="id">
                    <label for="id"></label> <input type="hidden" name="user_id" required>
                </div>
            </div>
        </div>