<?php
 
$html = <<<EOF
<p>Hi Hello <a href="#">World</a></p>.
This is <div class="myclass">testing <a href="#">content</a>. some more content</div>.
One more <a href="#"> Link </a>.
EOF;
 
$dom = new DOMDocument();
$dom->loadHTML($html);
 
$xpath = new DOMXpath($dom);
 
$links = $xpath->query("//a[not(ancestor::div[@class='myclass'])]");
 
foreach ($links as $link) {
	$link->parentNode->removeChild($link);
}
 
// just to test it out
echo $dom->saveHTML();
?>

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