<?php
 
$html = <<<DATA
<a href="somelink.html" target="_blank"><img src="myimage.jpg" alt="alt" title="sometitle" /></a>
<a href="somelink1.php" target="_parent">link_no1</a>
<a href="somelink2.php">link_no2</a>
<a href="someimage.jpg"><img src="image2.png"></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('target', '_blank');
}
 
echo $dom->saveHTML();

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