WPMonthlyArchives.php

<!-- Monthly Archive -->
<div class="widget">
    <h4 class="monthly-archive-title"><i class="icon-calendar"></i> 月別アーカイブ</h4>
    <?php 
    $monthly_archives = wp_get_archives( 
        $args = array(
            'type' => 'monthly',
            'show_post_count' => true,
            'format' => 'custom', //added 220718
            'before' => '<li>',   //changed 220718
            'after' => '</li>,',  //changed 220718
            'echo' => 0,
        ));
    $monthly_archives = explode(',', $monthly_archives); //配列化
    array_pop($monthly_archives); //末尾の空白要素を削除
    //var_dump($monthly_archives);

    $yearly_archives = wp_get_archives(
        $args = array(
            'type' => 'yearly',
            'format' => 'custom',
            'before' => '',
            'after' => ',',
            'echo' => 0,
        ));
    $yearly_archives = explode(',', $yearly_archives); //配列化
    array_pop($yearly_archives); //末尾の空白要素を削除
    //var_dump($yearly_archives);

    $this_year = (string)idate('Y'); //今年4桁

    $out = '<ul class="archive-list">';

    foreach ($yearly_archives as $year) {
        $the_year = substr($year,-8,4);

        if ($the_year === $this_year):
            $out .= '<li class="year acv_open current">' . $the_year;
            $out .= '<ul class="month-archive-list">';
        else:
            $out .= '<li class="year">' . $the_year;
            $out .= '<ul class="month-archive-list">';
        endif;

            foreach ($monthly_archives as $month) {
                $pos = strpos($month, $the_year); //月毎アーカイブの文字列中に、ターゲットとなる年が含まれているか
                if ($pos !== false):
                    $out .= $month;
                endif;
            }
        $out .= '</ul>';
        $out .= '</li>'; // 閉じる <li class="year"> //position changed 220718
    }
    $out .= '</ul>'; // 閉じる <ul class="archive-list">

    echo $out;
    //var_dump($out);
    ?>
</div>
<!-- /Monthly Archive -->