- preg_replace: documentation ( source)
<?php
$content = '<img src="https://link.com">
https://link.com<img src=\'https://link.com\'>';
$acturl = '~(?<!src=["\'])(?:(https?)://([^\s<]+)|(www\.[^\s<]+?\.[^\s<]+))(?<![\.,:])~i';
$content = preg_replace($acturl, '<a href="$0">$0</a>', $content);
echo $content;