我如何使用php curl获得Facebook视频可下载链接

Question

<?php    
    //echo $string;

    function get_html($url) {
        $urlnormal = str_replace("https", 'http', $url);    
        $userAgent = 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)';

        $c = curl_init($urlnormal);
        curl_setopt($c,CURLOPT_RETURNTRANSFER, true);
        curl_setopt($c,CURLOPT_FOLLOWLOCATION,true);
        curl_setopt($c, CURLOPT_USERAGENT, $userAgent);
        curl_setopt($c, CURLOPT_SSL_VERIFYPEER, false);

        $html = curl_exec($c);
        $status = curl_getinfo($c, CURLINFO_HTTP_CODE);
        curl_close($c);
        return $html;
    }

    $start = "video&quot;,&quot;src&quot;:&quot;"; 
    $end =",&quot;width&quot;:320,&quot;height&quot;";
    $s= htmlspecialchars($start);
    $e=htmlspecialchars($end);

    function getBetween($string, $start, $end ){
        if (strpos($string, $start)) { // required if $start not exist in $string
            $startCharCount = strpos($string, $start) + strlen($start);
            $firstSubStr = substr($string, $startCharCount, strlen($string));
            $endCharCount = strpos($firstSubStr, $end);
            if ($endCharCount == 0) {
                $endCharCount = strlen($firstSubStr);
            }
            return substr($firstSubStr, 0, $endCharCount);
        } 
        else 
        {
            return '';
        }
    }
    $string =  get_html('https://m.facebook.com/1995740577407064/');
    echo getBetween($string,$s,$e);
?>

方法get_html()用于获取url的html，获取HTML之后，我使用getBetween()方法仅打印介于$start和$end之间的字符串。

当我只回显$ string时，链接会自动重定向到视频页面，但是当我回显getBetwwen()方法时，它什么也没显示，我尝试了很多次但没有用，请帮忙。