understanding pdo and oop - php

Question

I learning oop and want to use pdo to execute mysql query. I have a query inside function that I want to execute. When I do this I get an error:
Fatal error: Call to a member function exec() on a non-object
What I'am doing wrong?
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
function testDuplicate($model) {
$SQL = "SELECT product_id FROM " . DB_PREFIX . "product WHERE model LIKE '" .$model . "'";
$result = $conn->exec($SQL);
if ($result->rows) return false;
return true;
}
function testDuplicateCat($cat) {
$SQL = "SELECT category_id FROM " . DB_PREFIX . "category WHERE category_id = '" .$cat . "'";
$result = $conn->exec($SQL);
if ($result->rows) return false;
return true;
}
foreach ($xml->PRODUCT as $child) {
if(testDuplicate($child->ID)){
...
}
}

Answer 1

This issue is raised because the $conn variable inside the testDuplicate function is not defined inside the scope of the function.
You could do this:
function testDuplicate($model) {
global $conn;
...
}
However it is not advice able to do so, its better to use static variables.
function getconn(){
static $conn;
if(!isset($conn)){
$conn = new PDO(...);
}
return $conn;
}
function foobar(){
$result = getconn()->query($sql);
while($row = $result->fetch()){
$ids[] = $row['category_id'];
}
return sizeof($ids) > 0 ? $ids : false;
}
if(($list = foobar()) == false){
echo "products " . var_export($list) . ' are duplicate values';
}
Why? Because you cannot simply overwrite the connection variable, even by accident or by using someone elses code. There are better alternatives but this is just a quick and safe example.