PHP Class not Returning Result - php

Question

I am working on an Item Inventory Web App. I want users should be able to add and assign item to a user. Each user is entitled to one item at a time. If, say, user a already has an item assigned and you want to add more item, the system should lodge an error that will tell you
to withdraw the item be issuing a new one but the errors are not getting lodged in the error[] array even though it shows that the array is not empty. It only echo out the serial number a = 1 and a++ but the text is not there.
class.inc.php
class Summary {
public $result;
public $conn;
public $SQ;
public $q;
public $updateDB;
public $checkDB;
public $returned_result;
public $a;
public $data;
public $col;
public function __construct(){
$this->conn = new PDO('mysql:host=localhost; dbname=dB', 'root', '');
$this->conn->setAttribute(PDO:: ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
public function updateDB($coloumn, $data, $id){
$SQ = "UPDATE mytable SET $coloumn = ? WHERE staffID = ?";
$q = $this->conn->prepare($SQ) or die("ERROR: " . implode(":", $this->conn->errorInfo()));
$q->bindParam(1, $data);
$q->bindParam(2, $id);
if ($q->execute()){
$success = 'Record updated successfully';
};
return $success;
}
public function checkDB($col, $data){
$status = 'Active';
$SQ = "SELECT surname FROM mytable WHERE $col = ? AND status = ?";
$q = $this->conn->prepare($SQ) or die("ERROR: " . implode(":", $this->conn->errorInfo()));
$q->bindParam(1, $data);
$q->bindParam(2, $status);
$q->execute();
if($result = $q->fetch(PDO::FETCH_BOTH)){
$a = $result[0];
if ($a == ''){
$this->returned_result = 'N';
}
else {
$this->returned_result = "This item (". $data . ") is in use by ". $a . ". Please widthraw the item";
}
}
return $this->returned_result;
}
}
index.php:
include('class.inc.php');
$summary = new Summary;
$error = array();
if(isset($_POST['saveRecord']) ) {
$system_name = strtoupper ( $_POST['system_name'] );
$result =$summary->checkDB('systemName', $system_name); //check if the item is in use
if ( $result == 'N' ){
$summary->updateDB('systemName', $system_name, $id);
$update = $summary->updateDB;
}
else $error[] = $result;
$system_serial_number = strtoupper ( $_POST['system_serial_number'] );
$result =$summary->checkDB('CPUSerial', $system_serial_number); //check if the item is in use
if ( $result == 'N' ){
$summary->updateDB('CPUSerial', $system_serial_number, $id);
$update = $summary->updateDB;
}
else $error[] = $result;
}
if(isset($_POST['saveRecord']) && !empty( $error ) ) {
echo "<div class = 'text-error'>";
$a = 1;
foreach ($error as $err){
echo '<p>' . $a . '. ' .$err . '</p>';
$a++;
}
echo "</div>";
}
Any help will be greatly appreciated. And what am I doing wrong with regards to OOP way of programming?