Laravel：用户定义的MySQL变量在Laravel中总是为NULL，但在MySQL CLI中不是

I have a query with a nested select that uses the result of the first select. I have the query working perfectly as I tested using Sequel Pro's query checker. It returns my desired results. When I pass this query over to Laravel and run a test on it using a simple echo statement, it outputs NULL.

Here is my PHP code and query:

$crons = DB::select('
    SELECT @quiet_crons:=ROUND(SUM(TIME_TO_SEC(TIMEDIFF(`end_time`, `start_time`))/60/15))
    AS quiet_crons,
    FLOOR(SUM(TIME_TO_SEC(TIMEDIFF(TIME('24:00'), TIME(NOW())))/60/15-@quiet_crons))
    AS remaining_crons
    FROM `quiet_periods`
    WHERE `day` = DAYNAME(NOW())
');

And this is the output from the console:

array(1) {
  [0]=>
  object(stdClass)#689 (2) {
    ["quiet_crons"]=>
    string(2) "24"
    ["remaining_crons"]=>
    NULL
  }
}

The remaining_crons result should never be NULL.

Please Note: I saw an article on here that talked about user-defined MySQL variables in Laravel and I tried the solution yet it did not solve my problem. This is the link I am referring to: User-defined MySQL variables in Laravel 3?

MySQL user defined variables are a bit tricky. From the manual;

As a general rule, other than in SET statements, you should never assign a value to a user variable and read the value within the same statement. For example, to increment a variable, this is okay:

SET @a = @a + 1;

For other statements, such as SELECT, you might get the results you expect, but this is not guaranteed. In the following statement, you might think that MySQL will evaluate @a first and then do an assignment second:
...

You can rewrite the statement without the variable though;

SELECT quiet_crons, 
  FLOOR(SUM(TIME_TO_SEC(TIMEDIFF(TIME('24:00'), TIME(NOW())))/60/15-quiet_crons))
  AS remaining_crons
FROM (
  SELECT ROUND(SUM(TIME_TO_SEC(TIMEDIFF(`end_time`, `start_time`))/60/15))
  AS quiet_crons
  FROM `quiet_periods`
  WHERE `day` = DAYNAME(NOW())
) a;

An SQfiddle to test with.

Okay, it is a bit weird but as soon as I SET (initialise) the variable first in a separate query, the resulting query works fine in Laravel.

Here is my code that I added above the previous code:

$crons = DB::select(DB::raw('
    SET @quiet_crons = 0;
'));

Maybe someone could explain why this works the way it does in Laravel when MySQL just does it's thing and works anyway...