<?php
$codes = array('tn','us','fr');
$names = array('Tunisia','United States','France');
$countries = array_combine($codes, $names);
foreach($countries as $code=>$name) {
     echo '<option value="' . $code . '">' . $name . '</option>';    
}

<option value="tn">Tunisia</option><option value="us">United States</option><option value="fr">France</option>