your link to inventory.php has a parameter ?id=$id
assuming $id is the id link of the record in the database
that id is available in inventory.php as $id
somewhere in the inventory.php file there should be (could be, usually is) an sql query that selects the row defined by that id
something like
"select * from database where id=$id"
I could not find such query in the file
I could find many queries, but none that selected on the basis of $id
it may be as simple as finding the variable used in inventory.php to select and
change that named variable to $id /* or */
change the parameter name passed by the link to that used in inventory.php /* or */
add a sql select with the right format
the sql in the files is convoluted, almost as if written to hide its intent