コード
すべての$menu値:class="*"をclass="current"に置き換えます
$menuA = preg_replace('~class=".+"~', 'class="current"', $menu);
print_r($menuA);
すべての$menu値:className"current"を現在のクラスに追加します
$menuB = preg_replace('~class="(.+?)"~', 'class="$1 current"', $menu);
print_r($menuB);
選択したメニュー:class="*"をclass="current"に置き換えます
$menuC = preg_replace('~class=".*?page-item-'.$selectedMenuId.'"~', 'class="current"', $menu);
print_r($menuC);
選択したメニュー:className"current"を現在のクラスに追加します
$menuD = preg_replace('~class="(.*?)page-item-('.$selectedMenuId.')"~', 'class="$1page-item-$2 current"', $menu);
print_r($menuD);
出力
$menuA = Array
(
[0] => <li class="current">
[1] => <li class="current">
[2] => <li class="current">
[3] => <li class="current">
[4] => <li class="current">
)
$menuB = Array
(
[0] => <li class="page_item page-item-155 current">
[1] => <li class="page_item page-item-49 current">
[2] => <li class="page_item page-item-72 current">
[3] => <li class="page_item page-item-18 current">
[4] => <li class="page_item page-item-50 current">
)
$menuC = Array
(
[0] => <li class="page_item page-item-155">
[1] => <li class="page_item page-item-49">
[2] => <li class="current">
[3] => <li class="page_item page-item-18">
[4] => <li class="page_item page-item-50">
)
$menuD = Array
(
[0] => <li class="page_item page-item-155">
[1] => <li class="page_item page-item-49">
[2] => <li class="page_item page-item-72 current">
[3] => <li class="page_item page-item-18">
[4] => <li class="page_item page-item-50">
)