// query parameters
$p[0] = 'id';
$p[1] = 'name';

// query database
$result = $dal->selectFieldsOrder('inventory_categories', $p, 'name asc');

$numRows = $result->num_rows;
$loopCount = 1;

$json = '{ '
      . '"categories" : [';

// build json string
while ($row = $result->fetch_assoc()) {
    $json .= '{ '
          . '"id" : "' . $row['id'] . '", '
          . '"name" : "' . addcslashes($row['name'], '"') . '" '
          . '}';

    // add comma if       
    if ($loopCount < $numRows) {
        $json .= ', ';
        $loopCount++;
    }
}

$json .= '] '
      . '}';

echo $json;