The reason you're getting the error, is because you are not building your echo statement correctly.
Instead of doing...
echo '<option value=\"{$row['Omschrijving']}\">{$row['Omschrijving']}</option>';
You should do:
echo '<option value="{' . $row['Omschrijving'] . '}">{' . $row['Omschrijving'] . '}</option>';
The server will then parse the echo statement to html like follows:
<option value="{whatever value}">{whatever value}</option>
Hope that helps!