如何使用mdb2 php从数据库中选择特定字段？

I'm trying to select a specific ID that is auto-generated by the oracle database for a specific session user(from log in and its working fine). I'm using MDB2 to achieve this. I am following the pear: MDB2 manual and did the codes the way it is but I'm getting an error. Please help.

I have used the fetchOne(MDB2_FETCHMODE_ASSOC) to achieve this from the reading manual and its giving an error

<?php   
session_start();
$user = $_SESSION['user'];
echo "Welcome:\t".$user;
require_once 'MDB2.php';//pear MDB2
include "conn.php";
 $do= "SELECT customer_id FROM customer WHERE username=".$user;
 $query= $db->query($do);
 if($one=$query->fetchOne(MDB2_FETCHMODE_ASSOC)){
    $id= $one['customer_id'];
echo ($id);
}
?>

The number 2 should be printed out but instead there this error: Fatal error: Call to undefined method MDB2_Error::fetchOne() in C:\wamp64\www\grahams\home.php on line 12

Try using below code.

<?php   
session_start();
$user = $_SESSION['user'];
echo "Welcome:\t".$user;
require_once 'MDB2.php';//pear MDB2
include "conn.php";
    $do= "SELECT customer_id FROM customer WHERE username=".$user;
    $id = queryOne($do);
    if(isset($id))
    {
        echo $id;
    }
?>

Since queryOne() query the database and select the column from the given query.

Edit: in conn.php check whether the connection is okay. following is the example code.

<?php

$dsn = array(
    'phptype'   => 'oci8',
    'hostspec'  => null,
    'username'  => 'oci_username',
    'password'  => 'oci_password',
);

// Establish database connection
$db = MDB2::connect($dsn);

// Check if the connection was established successfully

if (PEAR::isError($db)) {
    die('Could not create database connection. "'.$db->getMessage().'"');
}

?>