<?php

$regexp = '#<h2>Related Anime</h2>(.+?)<h2>#s';
$html = '
<h2>Related Anime</h2>Adaptation: <a href="/manga/418/Mushishi">Mushishi</a><br>Sequel: <a href="/anime/21329/Mushishi_Special:_Hihamukage">Mushishi Special: Hihamukage</a>, <a href="/anime/21939/Mushishi_Zoku_Shou">Mushishi Zoku Shou</a><br>
		<br />
		<h2>
';
preg_match($regexp, $html, $dig);

var_dump($dig);