<?php
echo "<select name='number'>";
for ($i = 100; $i <= 999; $i++) {
    echo "<option value='$i'>$i</option>";
}
echo "</select>";
 
 
echo "<select name='number'>";
$i = 100;
while ($i <= 999) {
    echo "<option value='$i'>$i</option>";
$i++;
}
echo "</select>";
?>