Why setting the group_concat_max_len variable in a query results in an error in PHP's bind_param()?

Question

I have the following query:

$query = <<<SQL
SELECT
      year,  
      count(*) AS `counter`, 
      GROUP_CONCAT(team) AS `team_list` 
    FROM
      team_list 
    WHERE year IS NOT NULL              
SQL;

if (!empty($sql)) { //$sql is an array of SQL WHERE statements "a IN (a,b,c)"
    $query .= ' AND ' . implode(' AND ', $sql);
} 
    

    $query .= 'GROUP BY year ORDER BY year';
    
    
    /////////////////////////////
    //EXECUTING THE QUERIES
    /////////////////////////////
    
    //Filter count to know how many 's' variable have to be bind to the prepared statement
    $filterCount = count($teams) + count($countries) + count($years) + count($rankings); //These are my ajax elements that are also used in the $sql variable
        
    //Data query
    $queryYears = $connection->prepare($query);
    $queryYears->bind_param(str_repeat('s', $filterCount), ...$teams, ...$countries, ...$years, ...$rankings);
    $queryYears-> execute();

This all works very fine!

THE PROBLEM

However, once I try to enter SET SESSION group_concat_max_len = 1000000; at the beginning of my query statement I get the following error:

Fatal error: Uncaught Error: Call to a member function bind_param() on boolean

I understand that something is now wrong with my query, but when copy-pasting it to my DBMS the query can be executed without a problem.

What am I doing wrong here?

Answer 1

Your problem is that you are trying to execute two queries at once, and mysqli::prepare doesn't support that, so it fails and returns false. Instead, run the variable set as a separate query first:

$connection->query("SET SESSION group_concat_max_len = 1000000;") or die($connection->error);
$queryYears = $connection->prepare($query) or die($connection->error);
// etc.

Note that you should be checking the status of your calls, as I have done in the code above.