Method convertRelativeToAbsoluteUrls

/**
 * Convert relative (links, images sources) to absolute urls so that content is accessible in email
 *
 * @param   string  $content  The content need to convert
 *
 * @return  string  The converted content which the relative urls are converted to absolute urls
 *
 * @since  4.1.0
 */
public static function convertRelativeToAbsoluteUrls($content)
{
    $siteUrl = Uri::root();
    // Replace none SEF URLs by absolute SEF URLs
    if (strpos($content, 'href="/apis/2623-convertrelativetoabsoluteurls?')_!==%20false)%20{_%20%20%20%20%20%20%20%20preg_match_all('#href="index.php\\?([^"]+)"#m', $content, $matches);
        foreach ($matches[1] as $urlQueryString) {
            $content = str_replace('href="/apis/2623-convertrelativetoabsoluteurls?'___$urlQueryString___'="', 'href="' . Route::link('site', 'index.php?' . $urlQueryString, Route::TLS_IGNORE, true) . '"', $content);
        }
        self::checkContent($content);
    }
    // Replace relative links, image sources with absolute Urls
    $protocols = '[a-zA-Z0-9\\-]+:';
    $attributes = array('href=', 'src=', 'poster=');
    foreach ($attributes as $attribute) {
        if (strpos($content, $attribute) !== false) {
            $regex = '#\\s' . $attribute . '"(?!/|' . $protocols . '|\\#|\')([^"]*)"#m';
            $content = preg_replace($regex, ' ' . $attribute . '"' . $siteUrl . '$1"', $content);
            self::checkContent($content);
        }
    }
    return $content;
}