json file values are not returned in php

I have the following function, which returns the values contained in the file conf.json, which I verified that it exists, that has values inside, and that the route is correct.

public function getJson(){
    $get = file_get_contents('conf.json');
    return json_decode($get, true);
}

But using it this way does not return anything:

$ s = getJson (); echo $ s ['domain'];

There is no error, but it does not return anything either, and I verified that it has the correct values, this is the json file:

{     'domain': 'domain',     'user': 'user' }

I already verified that everything was in the right place and that it was well defined, but still does not take anything out when calling the function, and when calling "domain"