PHP插入问题'id'

Question

I'm trying to resolve an issue with an app. Originally we had problems due to PHP upgrading from PHP5 to PHP7. This seems to have been corrected in places, but now I am having problems again.

Can someone please check the syntax to check if it is inputting right?

if($_POST['create_special_appointment']){ 
    $customer_id   = htmlspecialchars($_POST['customer'], ENT_QUOTES);
    $category_id   = htmlspecialchars($_POST['category'], ENT_QUOTES);
    $start_date    = htmlspecialchars($_POST['start_date'], ENT_QUOTES);
    $end_date      = htmlspecialchars($_POST['end_date'], ENT_QUOTES);
    $start_time = htmlspecialchars($_POST['start_time'], ENT_QUOTES);
    $end_time   = htmlspecialchars($_POST['end_time'], ENT_QUOTES);
    $booked_for =  post_array_to_explode($_POST['booked_for']);
    $notes      = htmlspecialchars($_POST['notes'], ENT_QUOTES);
    $session_id = htmlspecialchars($_POST['session_id'], ENT_QUOTES);
    $time       = date("Y-m-d H:i:s");
    $agent_id   = get_agent_from_session($session_id, 'id');

    //if(!($start_time)){$start_time = '00:00';}
    //if(!($end_time)){$end_time = '00:00';}
    //$start_date = $start_date.' '.$start_time;
    //$end_date = $end_date.' '.$end_time;

    $date=date('Y-m-d',strtotime($date));
    $start_date = $start_date.' '.$start_time.":00";
    $end_date = $end_date.' '.$end_time.":00";

    //$start_date = format_datetime_mysql($start_date);
    //$end_date = format_datetime_mysql($end_date);

    $insert = $dbh->exec("INSERT INTO calendar_special(start_date, end_date, notes, category_special_id, added_by, booked_for, date_added, customer_id) VALUES ('$start_date', '$end_date', '$notes', '$category_id', '$agent_id', '$booked_for', '$time', '$customer_id')");
    if($insert){
        $notification_msg[] = array(type=>'alert-success', h4=>'Success!', msg=>'Appointment Created');
        header('Location: ' . $_SERVER['HTTP_REFERER']);
    }else{
        $notification_msg[] = array(type=>'alert-warning', h4=>'Warning!', msg=>'Appointment NOT Created');
        mysqli_error($con);
        var_dump($_REQUEST);
        print_r($dbh->errorInfo());
    }
    //temp-code
    //var_dump($insert);
}   

Actual form code is:

<select name="customer" id="customer-single" style="width:100%"> 
        <option value="">Select a customer (Optional)</option>
        <?php  foreach(customer_array($ajax['0']) as $customer){ ?>
            <option value="<?php echo $customer['id'] ?>">
        <?php echo $customer['business_name'] ?> - <?php echo $customer['first_name'] ?> <?php echo $customer['last_name'] ?></option>
        <?php } ?>
</select>

And the error I'm getting is:

---

"Warning! Appointment NOT Created"

array(11) { ["session_id"]=> string(64) "c5ec66786328e97af8f8fb6771912b58c5869986816b3d2bc9916b7e740cff44" ["create_special_appointment"]=> string(4) "true" ["customer"]=> string(0) "" ["category"]=> string(1) "1" ["start_date"]=> string(10) "2018-10-23" ["start_time"]=> string(5) "10:35" ["end_date"]=> string(10) "2018-10-31" ["end_time"]=> string(5) "10:35" ["booked_by"]=> string(1) "5" ["booked_for"]=> array(1) { [0]=> string(2) "71" } ["notes"]=> string(18) " Paul Holiday Test" } Array ( [0] => 22007 [1] => 1366 [2] => Incorrect integer value: '' for column 'customer_id' at row 1 )

---

Apologies if this question has been worded incorrectly, and if so, please advise.