- preg_replace: documentation ( source)
<?php
$str = '<head><script src="//s.ytimg.com/yts/jsbin/scheduler-vflcamh30/scheduler.js" type="text/javascript" name="scheduler/scheduler"></script>
<link rel="stylesheet" href="//s.ytimg.com/yts/cssbin/www-core-2x-webp-vfl3iP9Qf.css" name="www-core">
<link rel="stylesheet" href="//s.ytimg.com/yts/cssbin/www-home-c4-2x-webp-vfl_GJYCY.css" name="www-home-c4">
<link rel="stylesheet" href="//s.ytimg.com/yts/cssbin/www-pageframe-2x-webp-vflE-hEYP.css" name="www-pageframe">
<link rel="stylesheet" href="//s.ytimg.com/yts/cssbin/www-guide-2x-webp-vflZfXyUk.css" name="www-guide">
<title>YouTube</title><link rel="search" type="application/opensearchdescription+xml" href="https://www.youtube.com/opensearch?locale=en_US" title="YouTube Video Search"><link rel="shortcut icon" href="https://s.ytimg.com/yts/img/favicon-vflz7uhzw.ico" type="image/x-icon"> <link rel="icon" href="//s.ytimg.com/yts/img/favicon_32-vfl8NGn4k.png" sizes="32x32"><link rel="icon" href="//s.ytimg.com/yts/img/favicon_48-vfl1s0rGh.png" sizes="48x48"><link rel="icon" href="//s.ytimg.com/yts/img/favicon_96-vfldSA3ca.png" sizes="96x96"><link rel="icon" href="//s.ytimg.com/yts/img/favicon_144-vflWmzoXw.png" sizes="144x144"><link rel="canonical" href="https://www.youtube.com/"><link rel="alternate" media="handheld" href="https://m.youtube.com/?"><link rel="alternate" media="only screen and (max-width: 640px)" href="https://m.youtube.com/?"><meta name="description" content="Share your videos with friends, family, and the world"><meta name="keywords" content="video, sharing, camera phone, video phone, free, upload"> <meta property="og:image" content="//s.ytimg.com/yts/img/yt_1200-vfl4C3T0K.png">
<meta property="fb:app_id" content="87741124305">
<link rel="publisher" href="https://plus.google.com/115229808208707341778">
<link rel="alternate" href="android-app://com.google.android.youtube/http/www.youtube.com/">
<link rel="alternate" href="ios-app://544007664/vnd.youtube/www.youtube.com/">
</head>';
$regx = '/(src|href|data[\-a-z]+)="([^"]*)"/i';
$out = preg_replace($regx,'https://s.ytimg.com',$str);
echo $out;
?>