/**
* 图片地址替换成压缩URL
* @param string $content 内容
* @param string $suffix 后缀
*/
function get_img_thumb_url($content="",$suffix="!c550x260.jpg")
{
$pregRule = "/<[img|IMG].*?src=[\'|\"](.*?(?:[\.jpg|\.jpeg|\.png|\.gif|\.bmp]))[\'|\"].*?[\/]?>/";
$content = preg_replace($pregRule, '<img src="${1}'.$suffix.'" style="max-width:100%">', $content);
return $content;
}

$content = '<a href="#" rel="external nofollow" rel="external nofollow" ><img class="center" src="https://xxx.com/styles/images/default.jpg"></a>'
.'<p><img class="center" src="https://img.xxx.com/images/219_Ig5eZI.jpg" style="max-width: 100%;"></p>';
$newct = get_img_thumb_url($content);
print_r($newct);