Почему возникает ошибка Warning: A non-numeric value encountered?

когда $session_id = 1; - все хорошо но $session_id = $_SESSION['test']; ничего не работает

    $session_id = $_SESSION["test"];
$sql = mysqli_query($db_conn, "SELECT * FROM `clicker_user_para` WHERE `id` =  $session_id");
$res = mysqli_fetch_array($sql);
$score = $res["click_value"];
$cursor = $res["cursor"];
$video_card = $res["video-card"];
$video_card_rack = $res["video-card-rack"];
$super_computer = $res["super-computer"];
$server = $res["server"];

$cursor_exp = explode(",", $cursor);
$video_card_exp = explode(",", $video_card);
$video_card_rack_exp = explode(",", $video_card_rack);
$super_computer_exp = explode(",", $super_computer);
$server_exp =explode(",", $server);

$cursor_sec = $cursor_exp[0] * $cursor_exp[2];
$video_card_sec = $video_card_exp[0] * $video_card_exp[2];
$video_card_rack_sec = $video_card_rack_exp[0] * $video_card_rack_exp[2];
$super_computer_sec = $super_computer_exp[0] * $super_computer_exp[2];
$server_sec = $server_exp[0] * $server_exp[2];

$socre_print = $score + $cursor_sec + $video_card_sec + $video_card_rack_sec + $super_computer_sec + $server_sec;
$socre_print = bcdiv($socre_print, 1, 3);

mysqli_query($db_conn, "UPDATE `clicker_user_para` SET `click_value` = 
        '$socre_print' WHERE `id` = $session_id");

$upgrade_data = array('score' => $score);

$upgrade_data_json = json_encode($upgrade_data);
$connection->send($upgrade_data_json);