SELECT count rows from 2 tables and sum the results

I have the PHP code:

$query = mysqli_query($mysqli, "SELECT * FROM `table_1`");
$result = mysqli_num_rows($query);

$queryTwo = mysqli_query($mysqli, "SELECT * FROM `table_2`");
$resultTwo = mysqli_num_rows($queryTwo);

$number =  $result + $resultTwo;
return $number;

The point is that sometimes, the $number variable is returning NULL, when it should not supposed to do that.

I have always rows in those 2 tables, and the returned result should not be NULL, ever.

Is this a correct approach to sum the number of rows from 2 tables? I don`t understand why sometimes I get NULL instead of a number.

Well, I would suggest you to do it like

  select ( select count(*) from Table1 ) + ( select count(*) from Table2 ) 
  as total_rows

executing this query and getting the value of total_rows will return you true result

Or you can create a stored procedure to do the same thing. as explained below

CREATE PROCEDURE sp_Test
AS
-- Create two integer values
DECLARE @tableOneCount int, @tableTwoCount int

-- Get the number of rows from the first table
SELECT @tableOneCount = (SELECT COUNT(*) FROM Table1
                         WHERE WhereClause)
SELECT @tableTwoCount = (SELECT COUNT(*) FROM Table2
                         WHERE WhereClause)

-- Return the sum of the two table sizes
SELECT TotalCount = @tableOneCount + @tableTwoCount