cdn.php

<?php
function getCdnUrls($html)
{
        $cdnUrl = "//cdn.mon-domain.com";
        $baseUrl = "mon-domain.com";

        $patterns = [
                // Match sources that are from the www or mobile urls
                // We check for the schema as well as protocol relative urls
            '~(<(img|script|link)[^>]*)(src|href) *= *(["\'])(https?:)?//(' . preg_quote($baseUrl) . ')~',

                // Match sources that just begin relative to the current domain
                // Our regex matches the initial "/" character with a negative lookahead
                // ensuring the next character is not a slash
            '~(<(img|script|link)[^>]*)(src|href) *= *(["\'])/(?!/)~'
        ];

        return $html = preg_replace_callback($patterns, function (array $matches) use ($cdnUrl) {
                return $matches[1] . $matches[3] .  "=" . $matches[4] . $cdnUrl;
        }, $html);
}


//Call the function to get your CDN URL:
$html = '<img src="https://mon-domain.com/logo.png" />';
echo getCdnUrls($html);

// you will get it
// <img src="//cdn.mon-domain.com/logo.png" />
?>