POnumber is an array. Probably you must do a for/whyle query.

Write: dd($POnumber) and show us

It only shows empty array: Array ( )

array(0) { }

Hi.
POnumber is not an array. It is obviously a string.
When making a query with numbers inside (or other stuff), instead fo the numbers you see only (?). This query with questionmarks is followed by an array. Every element in the array represents the value for a questionmark.
In your case, the array should be array(1){ 0 => "PO/DM/20:0012345"}
You are probably missing s '/' between the time and number.

$POnumber = "PO/DM/" . $time . "/". $number;

To make this more readable, in PHP you can do this

$POnumber = "PO/DM/$time/$number";

$PO contains the result. In your case the resulting empty. No matches found.