第1行的“ user_id”列的整数值不正确：“”

Question

I'm trying to insert into my SQL database. For some reason I'm getting the Incorrect integer value: '' for column 'user_id' at row 1 error.

Here's my table schema:

As you can see, user_id is a foreign key that references a column from another table. I'm not sure if you need that information but yeah. I'm getting the error from my insert php.

Here's my insert 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>"; } } ?> 

Form:

 <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> 

Answer 1

You never set a value to user_id

<input type="hidden" name="user_id" required>

so $_POST['user_id']; is blank

Answer 2

Added

 <div class="id"> <label for="id"></label> <input type="text" readonly="readonly" value="<? php echo $user_id; ?>" name="user_id" required> </div>