Easyphp works, linux server return Call to a member function on a non-object

I am using Easyphp for development with default settings. All my code works fine with connection on local Easyphp db. But when I upload it on my webhosting which is using linux server - all functions with objects suddenly are returing OOP errors. This is exp. of normal function I normaly write:

function user_data($user_id){
    global $db;
    $data = array();
    $user_id = (int)$user_id;

    $func_num_args = func_num_args();
    $func_get_args = func_get_args();

    if($func_num_args > 1) {
        unset($func_get_args[0]);
        $fields = '`' . implode('`, `', $func_get_args) . '`';
        $query = "SELECT $fields FROM users WHERE user_id = '$user_id'";
        $result = $db->query($query);
        $data = $result->fetch_assoc(); 
        return $data;
        $result->free();
        $data->free();
    }
}

Everything works fine on my local php server. After I upload it on webhosting with linux server I get " Call to a member function fetch_assoc() on a non-object error " on lines like :

 $data = $result->fetch_assoc();

Can anybody tell me what I am writing wrong?

DB connection for easyphp:

 $db = new mysqli("$DB_HOST","$DB_USER","$DB_PASSWORD","$DB_NAME");

DB connection for linux server:

 $socket = "/tmp/mysql51.sock";
 $db = new mysqli("$DB_HOST","$DB_USER","$DB_PASSWORD","$DB_NAME", 0, $socket);

Thank you

EDIT - SOLUTION

I found the reason for error. It is in writing variables in SQL query inside PHP code. Query should be like:

$query = "SELECT $fields FROM users WHERE user_id = '".$user_id."'";

Reason of error is still for me mistery. it can be platform or PHP versions.