<?php

$host = "localhost";
$db = "dev";
$user = "dev";
$pass = "dev";
$dsn = "mysql:host=$host;dbname=$db;";

try
{
    $pdo = new PDO($dsn, $user, $pass);
} catch (PDOException $ex) {
    echo 'Connection failed: ' . $ex->getMessage();
    die();
}

?>

<?php

include_once('pdo.inc.php');

class Customers
{
    function GetCustomerName($pdo, $customer_id)
    {
        $query = "SELECT first_name FROM Customers WHERE customer_id = :customer_id LIMIT 1";
        $stmt = $pdo->prepare($query);
        $stmt->bindParam(':customer_id', $customer_id);
        $stmt->execute();
        return $stmt->fetchColumn();
    }
}

?>

<?php

include_once('customers.class.php');

$a = new Customers();

echo $a->GetCustomerName($pdo, 1);

?>

function GetCustomerName(PDO $pdo, $customer_id) {

function GetCustomerName(PDO $pdo, int $customer_id) {

if (empty($customer_id) || !is_int($customer_id) || $customer_id < 1) {
    throw new InvalidArgumentException(
        'Send me a valid positive integer for $customer_id' .
        ' Value sent: ' . var_export($customer_id, true)
    );
}

<?php
define('DB_CONFIG_HOST', 'localhost');
define('DB_CONFIG_DB', 'dev');
define('DB_CONFIG_USER', 'dev');
define('DB_CONFIG_PW', 'dev');
$dsn = 'mysql:host=' . DB_CONFIG_HOST . ';dbname=' . DB_CONFIG_DB . ';';
define('DB_CONFIG_DSN', $dsn);l
?>

<?php
class Customer
{
    function getCustomerName(PDO $pdo, $customer_id)
    {
        // validate parameters
        // in PHP 7 you would just add int type hinting in method signature
        // and could change the conditional below to only look
        // positive integer values assuming the id is > 0
        if (empty($customer_id) || !is_int($customer_id) || $customer_id < 1) {
            throw new InvalidArgumentException(
                'Send me a valid positive integer for $customer_id' .
                ' Value sent: ' . var_export($customer_id, true)
            );
        }

        // query DB for customer name
        $query = "SELECT first_name
                  FROM Customers
                  WHERE customer_id = :customer_id
                  LIMIT 1";
        try {
            $stmt = $pdo->prepare($query);
            $stmt->bindParam(':customer_id', $customer_id);
            $stmt->execute();
            if($stmt->rowCount === 0) {
                // no match found
                return false;
            } else {
                return $stmt->fetchColumn();
            }
        } catch (PDOException $e) {
            error_log(
                'Something went wrong with query. Message: ' .
                $e->getMessage();
            );
            // you could also rethrow exception here instead of returning null
            // if you want to abort method completion and have caller handle exception.
            // or of course not use try-catch block here at all
            // if you don't want any extra exception handling in this method.
            return null;
        }
    }
}

?>

<?php


include_once('pdoConfig.inc.php');
include_once('customers.class.php');

// instantiate PDO object
try
{
    $pdo = new PDO(DB_CONFIG_DSN, DB_CONFIG_USER, DB_CONFIG_PW);
} catch (PDOException $e) {
    error_log('Connection failed with: ' . $e->getMessage());
    die();
}

// instantiate customer object
// you might want to include in try-catch if instantiation is expected
// to potentially throw exceptions
$a = new Customer();

try {
    echo $a->getCustomerName($pdo, 1);
} catch (Exception $e) {
    error_log('Customer name lookup failed with: ' . $e->getMessage());
}
?>