What problem are you trying to solve? Are you trying to figure out which ID you will need to use when inserting a record? In later versions (you probably want 2.3.4 or greater), SQLite will insert the ID for you if you try to insert a NUL value. If you need to find out what ID was inserted, just call the new (as of 2.2.3) sqlite_last_insert_rowid() function.

Also, you have a security hole in that code. Please read up about SQL injection attacks. You can fix the security hole using placeholders.

    $sth = $dbh->prepare( "select CODE from INV where CODE = ?");
    # Execute the SQL statement and generate a result set
    $sth->execute($user_input);
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