<?php $set_of_numbers = "1,2,4,5,9"; $numbers = explode(",", $set_of_numbers); $range = range(1,12); $numbers_to_output = array_diff($range, $numbers); // [3,6,7,8,10,11,12] foreach($numbers_to_output as $n){ echo '<option value='.$n.'>'.$n."</option>\n"; }
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