html5视频标签不使用php在chrome上播放

I am trying to download an mp4 video file in PHP. the file generated successfully with the code below.

$result_ad // is the PHP variable contains video content.

header("Content-Type: video/mp4");  
header("Content-Length: ".strlen($result_ad));

echo $result_ad;exit;

It generate an html5 video and successfully played it on Mozilla.

But not playing on google Chrome.