You are still wide open for an injection attack, since the value inserted in the selection box can be easily changed by the end user.
If you have a good side to the verification server, then doing this without a prepared statement will work.
With good, I mean something like this:
$array = Array("all", "your", "possible", "values", "from", "Select boxes"); if(in_array ($_POST['selectbox'], $array)){
Directly inserting user input is NEVER a good idea. You should never trust the end user!
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