Variable becomes empty inside scope loop (Octobercms) - php

Question

I have $slug variable that takes frontend url slug value and sends it to model scope. Inside scope if I use echo $slug it works and also it works inside query (I have it to display products that have only same category slug).
Query where $slug works:
return $query->orwhereHas('category', function($q) use ($slug){
$q->where('slug', '=', $slug);
})->paginate($perPage, $page);
Here is the SQL:
select * from `slasher_farming_mods` where exists (select * from `slasher_farming_categories` where `slasher_farming_mods`.`category_id` = `slasher_farming_categories`.`id` and `slug` = 'medium-tractors') limit 4 offset 0
As visible slug value is "medium-tractors".
If I try to use $slug inside sort loop or inside brands loop it becomes empty
public function scopeListFrontEnd($query, $options = [], $slug = ''){
extract(array_merge([
'page' => 1,
'perPage' => 4,
'sort' => 'brandName desc',
'brands' => null,
'categorySlug' => null,
], $options));
print $slug; //This works here
if(!is_array($sort)) {
$sort = [$sort];
}
foreach ($sort as $_sort){
if(in_array($_sort, array_keys(self::$allowedSortingOptions))){
$parts = explode(' ', $_sort);
if(count($parts) < 2){
array_push($parts, 'desc');
}
list($sortField, $sortDirection) = $parts;
$query->orderBy($sortField, $sortDirection);
}
}
if($brands !== null) {
if(!is_array($brands)){
$brands = [$brands];
}
foreach ($brands as $brand){
$query->orwhereHas('brand', function($q) use ($brand){
$q->where('brand_id', '=', $brand);
});
}
}
$lastPage = $query->paginate($perPage, $page)->lastPage();
if($lastPage < $page) {
$page = 1;
}
return $query->orwhereHas('category', function($q) use ($slug){
$q->where('slug', '=', $slug);
})->paginate($perPage, $page);
}
Example SQL when using brands filter
select * from `slasher_farming_mods` where exists (select * from `slasher_farming_brands` where `slasher_farming_mods`.`brand_id` = `slasher_farming_brands`.`id` and `brand_id` = '5') or exists (select * from `slasher_farming_categories` where `slasher_farming_mods`.`category_id` = `slasher_farming_categories`.`id` and `slug` = '') order by `name` desc limit 4 offset 0
Slug variable becomes empty.
How could I keep slug variable data inside brand loop or inside sort loop?