Why my values of the form input does not pass to database

Currently I am working on an apply form that staff taking leave. The form is user need to fill on their leaveDuration, leaveDate and reason only. I need to pass the values that they fill in the input to database so I can retrieve. After the user done submit, there will be a message said submitted and redirect to home.php. This problem had stucked me for few days already… Thanks for helping me and I appreciate on your help…

This is the code that I currently have problem passing the values into the database called leave_management The functions that I place at the top of the form.php

<?php 
    $db = mysqli_connect("localhost", "root", "", "leave_management");
    if($db === false)
    die("ERROR: Could not connect. ");
    if(isset($_POST['submit_btn']))
    {
        $ID             =  $_SESSION['user']['ID'];
        $leaveType      =  $_POST['leaveType'];
        $leaveDuration  =  $_POST['leaveDuration'];
        $dateFrom       =  $_POST['dateFrom'];
        $reason         =  $_POST['reason'];
        $status;
        $query = "INSERT INTO form (ID, formID, leaveType, leaveDuration, dateFrom, reason, status, dateApprove, approveBy, remark, Created, Modified) 
        VALUES ('$ID', NULL, '$leaveType', '$leaveDuration', '$dateFrom', '$reason', 'Pending', '', NULL, '', '', '')";
        //mysqli_query($db, $query);
        if(mysqli_query($db ,$query)){
            echo "Records inserted successfully.";
        } else{
            echo "ERROR";
        }
        header("Location: home.php");
    }   
?>

This is the form that user need to fill

            <form role="form" method="post" action="form.php">
                <div class="form-group">
                    <label for="employeeName">
                        Employee Name:  
                    </label>
                    <input type="text" class="form-control" value="<?php echo $_SESSION['user']['name']?>" id="employeeName" name="name" disabled />
                </div>
                <div class="form-group">
                    <label for="ICno">
                        IC Number:  
                    </label>
                    <input type="text" class="form-control" value="<?php echo $_SESSION['user']['IC']?>" id="ICno" name="IC" disabled />
                </div>
                <div class="form-group">
                    <label for="selectLeave">
                        Apply Leave for: &ensp; 
                    </label>
                    <select name="leaveType" required>
                        <option value="Annual Leave">Annual Leave</option>
                        <option value="Unpaid Leave">Unpaid Leave</option>
                        <option value="Sick Leave">Sick Leave</option>
                    </select>
                </div>
                <div class="form-inline">
                    <label for="duration">
                        Duration:  &ensp;
                    </label>
                    <input type="number" class="form-control" id="duration" name="leaveDuration" required /> &ensp;day(s)
                </div>
                <br>
                <div class="form-inline">
                    <label for="dateFrom">
                        Date from:  &ensp;
                    </label>
                    <input type="date" class="form-control" id="datetimepicker" name="dateFrom" required /> 
                </div>
                <br>
                <div>
                    <label for="reason">
                    Reason:  
                    </label>
                    <textarea name="reason" class="form-control" id="reason" rows="5" cols="40" required></textarea>
                </div>
                <br>
                <div class="checkbox">
                    <label>
                        <input type="checkbox" required /> I had understand and agree with the <a href="protocol.php">Leave Office Protocol and Term and Condition</a>.
                    </label>
                </div> 
                <input type="submit" class="btn btn-primary" name="submit_btn"/>
            </form>

This is the database of the form.

It display the ERROR in echo “ERROR”…

Answer

The formID column you have made it auto-increment and not null in the table. And still, you are passing null to it in PHP code.

Also have a look at how to get the MySQL errors

https://www.w3schools.com/php/func_mysqli_error.asp

And your code is fully prone to SQL injection

Read about the PHP Prepared statements. https://www.w3schools.com/php/php_mysql_prepared_statements.asp

Note:

And then the columns to which you know can or cannot contain values sometimes, you can mark them as NULL in DB

Your query should be like:

$query = "INSERT INTO form (ID, leaveType, leaveDuration, dateFrom, reason, status) VALUES ('$ID','$leaveType', '$leaveDuration', '$dateFrom', '$reason', 'Pending')";