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<select name="m">
  <option value="" selected="selected">年月選択</option>
  <?php
  $archives_m = wp_get_archives('type=monthly&format=custom&before=&after=,&echo=0');
  $archive_m_arr = explode(',', $archives_m);
  array_pop($archive_m_arr);
  foreach( $archive_m_arr as $archive_month ) {
    $string = strip_tags(ltrim($archive_month));
    $label = $string;
    $string = str_replace("年","", $string);
    $string = str_replace("月","", $string);
    if (strlen($string) == 5) {
    $last = substr($string,4,1);
    $string = substr_replace($string, "0", 4) . $last;
  }
    echo '<option value="' . $string . '">' . $label . '</option>';
  }
  ?>
</select>