Transformare cod mysql in PDO sau MSQLi

$query = "SELECT id_bilet, COUNT(customer_id) FROM orders WHERE customer_id='3'"; 
    $result = mysql_query($query) or die(mysql_error());
          // Print out result
   while($row = mysql_fetch_array($result)){
	echo "Comenzi <b>". $row['COUNT(customer_id)']."</b><br/>";
        }
	// interogare sql SELECT 
$sql = "SELECT id, COUNT(customer_id) FROM orders WHERE customer_id='3'"; 

// executa interogarea si retine datele returnate
$result = $db->query($sql);

// daca $result contine cel putin un rand
if ($result->num_rows > 0) {
  // afiseaza datele din fiecare rand din $result
  while($row = $result->fetch_assoc()) {
  echo "Comenzi <b>". $row['COUNT(customer_id)']."</b><br/>";
  }
}
else {
  echo '0 rezultate';
}

$db->close();

Ok, deci folosesti mysqli

"SELECT id in al doilea post, iar in primul ai "SELECT id_bilet
WHERE customer_id='3' valoarea este hardcoded 3
Probabil ai nevoie de $customerId = $db->real_escape_string($customerId); sau $customerId = (int) $customerId;

if ($result = $db->query($sql)) {
...
} else {
// A avut loc o eroare, sau nu tratam varianta asta??
printf("Error: %s\n", $db->error);
}

$row['COUNT(customer_id)'] este asa urat, nu puteai sa pui in query COUNT(customer_id) as count_customer_id