<?php
 
$html = <<<DATA
<a href="http://w...content-available-to-author-only...e.com/a">Works</a>
<a href="http://w...content-available-to-author-only...e.com/a/b/c">Doesnt Work</a>
<a href="http://w...content-available-to-author-only...e.com/x/y/z">Works</a>
DATA;
 
$dom = new DOMDocument('1.0', 'UTF-8');
$dom->loadHTML($html, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
 
$xpath = new DOMXPath($dom);
$links = $xpath->query('//a');
 
foreach($links as $link) { 
   $link->setAttribute('href', 'https://w...content-available-to-author-only...e.net/newlink/');
}
echo $dom->saveHTML();

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