How to encrypt value of DB query in Laravel

You are currently using the md5 encrypt function on the string collections.id, so you are not encrypting the result of the query but the database column name, which is why you are getting that the column doesn't exist.

By the naming of the $collection variable, I would presume that you are only expecting one result, if so you can do as below otherwise you will have to loop through each row and encrypt the column value. It's not the nicest way of doing it, it would be much nicer to use casting in a model class if you use models. You can read more about casting in the Laravel documentation https://laravel.com/docs/8.x/eloquent-mutators

Also, a note, if you are only expecting one row, use first() rather than get(), and if you are expecting several rows, use the variable name $collections as it makes the code easier to read as it tells readers that the query will return several collections rather than just one

$collection = DB::table('collections')
  ->select(['collections.id', 'collections.name', 'collections.category_id', 'users.name as username', 'collections.price', 'collections.image'])
  ->join('users','users.id','collections.user_id')
  ->first()
  ->toArray();
  
  if ($collection) {
    $collection['collections.id'] = md5(encrypt('collections.id'));
  }
  
  return response()->json([
    'collection'    => $collection,
  ], 200);