Mysql / PDO - Setting Variable - mysql

Question

I'm having trouble trying to set a variable then use it in a select statement. I keep getting a "general error" and can't figure out what I'm doing wrong. Any input would be appreciate. I'm trying to set a variable using subqueries with named parameters.
$query = $dbh->prepare("Set #available = (SELECT SUM(payments) FROM payments WHERE customer = :customer) - (SELECT SUM(charges) FROM charges WHERE customer = :customer); SELECT #available");
$query->bindParam(":customer", $customer);
$query->execute();

Answer 1

If you want to use MySQL user variables, for some reason, you don't need multi-queries support. They (user variables) live as long as you session (connection) is open. Therefore you can do something like this
$customer = 1;
try {
$db = new PDO('mysql:host=localhost;dbname=dbname;charset=UTF8', 'user', 'password');
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$sql = "SET #available = (SELECT SUM(payments) FROM payments WHERE customer = ?) -
(SELECT SUM(charges) FROM charges WHERE customer = ?)";
$query = $db->prepare($sql);
$query->execute(array($customer, $customer));
$query = $db->prepare("SELECT #available");
$query->execute();
$result = $query->fetchAll(PDO::FETCH_ASSOC);
} catch (PDOException $e) {
echo "Exeption: " .$e->getMessage();
$result = false;
}
$query = null;
$db = null;
var_dump($result);
Sample output:
array(1) {
[0]=>
array(1) {
["#available"]=>
string(6) "100.00"
}
}

Answer 2

PDO doesn't seem to offer any formalised support for multi-queries (but some support does seem to be available), but mysqli does. Neither offer support for prepared multiple statements, though.
You can use mysqli like this:
$mysqli = new mysqli('servername', 'username', 'password', 'dbname');
$query = sprintf("Set #available = (SELECT SUM(payments) FROM payments WHERE customer = %1$s)" .
" - (SELECT SUM(charges) FROM charges WHERE customer = %1$s);".
" SELECT #available",
$mysqli->real_escape_string($customer) );
// Following code lifted from PHP Manual.
// This code will read multiple results, if they're available.
// Your query only returns one.
/* execute multi query */
if ($mysqli->multi_query($query)) {
do {
/* store first result set */
if ($result = $mysqli->store_result()) {
while ($row = $result->fetch_row()) {
printf("%s\n", $row[0]);
}
$result->free();
}
/* print divider */
if ($mysqli->more_results()) {
printf("-----------------\n");
}
} while ($mysqli->next_result());
}
The reference is here