echo json_encode(R::getAll('SELECT * FROM `category` WHERE `id_user` = 100'));
$id_user = 100;
echo json_encode(R::getAll('SELECT * FROM `category` WHERE `id_user` = $id_user'));
"SELECT * FROM `category` WHERE `id_user` = $id_user"
'SELECT * FROM `category` WHERE `id_user` = ' . $id_user