How to pass a variable to SQL query?

Question

PHP

The code works fine

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

How to replace 100 with a variable?

This returns an empty array

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

Answer 1

Who teaches you redbin, without even giving the basic syntax?
A variable is never passed directly to a request. It must be submitted separately.

echo  json_encode(R::getAll('SELECT * FROM `category` WHERE `id_user` = ?', [$id_user]));

Answer 2

A single-quoted string does not expand the variable.

Answer 3

Try SQL in double quotes " .

"SELECT * FROM `category` WHERE `id_user` = $id_user"

or

'SELECT * FROM `category` WHERE `id_user` = ' . $id_user