Cant access variable inside function

On a model I got a scope function with 3 parameters, $query, $sort_by and $sort_dir, I can access them before the join function, but for some reason I can't access inside the join function.

Why? I'm missing something?

public function scopeOrder($query, $sort_by=false, $sort_dir='ASC'){

    if(!$sort_by)
        return $query;

    var_dump($sort_dir); //prints 'ASC'

    return $query->join('product_translations', function($q){

        var_dump($sort_dir); //variable undefined <----- ERROR

        $q->on('product_products.id', '=', 'product_translations.product_id')->where('product_translations.locale', '=', 'de');

    })->select('product_products.*')->orderBy('product_translations.name');

}

via Chebli Mohamed