How to insert records to a many to many relationship table on an sql database using php?

I just started PHP and am encountering a problem with php where the sql command shown below does not execute (inserts records into a many to many relationship table) when calling it with php while it works perfectly when manually inserting it with mysql command line interface. I am using $ds and $da (manual inputs) instead of making use of sessions to make sure that the command is executed. Noting that I have other mysql queries and work perfectly fine when executed through php using the same database connection as the one shown hereunder.

$connectionStatua = connect_db();
mysqli_query($connectionStatus, $sql);

$username = $_SESSION["username"];
$ds = "dekna";
$da = "dsa.jpg";
$query="INSERT INTO `tbl_users_files` (`user_ID`, `file_ID`)
SELECT `u.id`, `f.id` FROM `users` as `u` CROSS JOIN `tbl_uploads` as `f`
WHERE `username` = '$ds' AND `file` = '$da'";
mysqli_query($connectionStatus, $query);