SET FOREIGN_KEY_CHECKS = 0不能正常工作

Question

My web application allow to get backups and set restore sql file.when restore script drop all the tables but some tables don't drop because there are foreign keys.so i added some code above of the restore.php file that will successfully execute in localhost but server it is not working.server not execute the SET FOREIGN_KEY_CHECKS = 0.

my local machine php version :5.38 Server machine php version :5.46

Thank you !!

here is the restore.php file

$remove_foreign_key = 'SET FOREIGN_KEY_CHECKS = 0';
$result = mysqli_query( $conn,$remove_foreign_key  );
if( $result ) {
}
else{
echo "Error
";
}


if (! empty($_FILES)) {
      // Validating SQL file type by extensions
    if (! in_array(strtolower(pathinfo($_FILES["backup_file"]["name"], PATHINFO_EXTENSION)), array(
        "sql"
    ))) {
        $response = array(
            "type" => "error",
            "message" => "Invalid File Type"
        );
    } else {
        if (is_uploaded_file($_FILES["backup_file"]["tmp_name"])) {



            move_uploaded_file($_FILES["backup_file"]["tmp_name"], $_FILES["backup_file"]["name"]);
            $response = restoreMysqlDB($_FILES["backup_file"]["name"], $conn);
        }
    }
}

function restoreMysqlDB($filePath, $conn)
{
    $sql = '';
    $error = '';

    if (file_exists($filePath)) {
        $lines = file($filePath);

        foreach ($lines as $line) {

            // Ignoring comments from the SQL script
            if (substr($line, 0, 2) == '--' || $line == '') {
                continue;
            }

            $sql .= $line;

            if (substr(trim($line), - 1, 1) == ';') {
                $result = mysqli_query($conn, $sql);
                if (! $result) {
                    $error .= mysqli_error($conn) . "
";
                }
                $sql = '';
            }
        } // end foreach

        if ($error) {
            $response = array(
                "type" => "error",
                "message" => $error
            );
        } else {
            $response = array(
                "type" => "success",
                "message" => "Database Restore Completed Successfully."
            );
        }
        unlink($filePath);
    } // end if file exists

    return $response;
}
?>