You need to assign keys to the items you're adding to the $ddmenu;
Eg. $ddmenu[$tablerow['idcategorieen']] = $tablerow['idcategorieen'];
will make the values be the idcategorieen.
Edit for clarification: If $ddmenu looks like this:
array(
1 => 'Books',
2 => 'Cats',
3 => 'Foo Bars'
)
The dropdown options will look like
<option value="1">Books</option>
<option value="2">Cats</option>
<option value="3">Foo Bars</option>