サンプルコード
<select name="year_month">
<?php
$current_year = date('Y');
$current_month = date('m');
for ($i = 0; $i < 24; $i++) {
$year = $current_year - floor(($current_month - 1 + $i) / 12);
$month = ($current_month - 1 + $i) % 12 + 1;
printf('<option value="%04d-%02d">%04d年%02d月</option>', $year, $month, $year, $month);
}
?>
</select>