//字符串
$test = '<embed src="/file/storage/media/20211210134527732898748.mp4" type="video/x-ms-asf-plugin" width="550" height="400" loop="true" />
<embed src="/file/storage/media/202112101345567b10f0628.mp4" type="video/x-ms-asf-plugin" width="550" height="400" autostart="true" loop="true" />
<embed src="/file/storage/media/202112101346276f2b76852.mp4" type="video/x-ms-asf-plugin" width="550" height="400" autostart="true" loop="true" />';
//正则查找embed
preg_match_all("/<embed.*?src=\"(.*?)\".* loop=\"true\" \/>/i",$test,$match);
dump($match);
//定义数组
$video = [];
foreach($match[1] as $k=>$v){
    $video[] = '<video src="'.$v.'" controls="" autoplay="" loop=""></video>';
}
dump($video);
//embed替换video
$data = str_replace($match[0],$video,$test);
echo $data;