<?php
$html = <<<EOF
this is the <a href='#'>test link</a> here and <a href='#'>there</a>.
EOF;
$dom = new DOMDocument();
$dom->loadHTML($html);
$atags=$dom->getElementsByTagName('a');
foreach($atags as $atag) {
$text = $atag->firstChild->cloneNode();
$r = $atag->parentNode->replaceChild($text, $atag);
}
echo $dom->saveHTML();
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
<html><body><p>this is the test link here and <a href="#">there</a>.</p></body></html>