Obtenga la suma de la columna MySQL en PHP

Puede manejarlo completamente en la consulta MySQL:

SELECT SUM(column_name) FROM table_name;

Usando PDO (mysql_query está en desuso)

$stmt = $handler->prepare('SELECT SUM(value) AS value_sum FROM codes');
$stmt->execute();

$row = $stmt->fetch(PDO::FETCH_ASSOC);
$sum = $row['value_sum'];

O usando mysqli:

$result = mysqli_query($conn, 'SELECT SUM(value) AS value_sum FROM codes'); 
$row = mysqli_fetch_assoc($result); 
$sum = $row['value_sum'];