如果你想要YouTube上某个特定视频ID的最大图片，那么URL应该是这样的：

http://i3.ytimg.com/vi/SomeVideoIDHere/0.jpg

使用API，您可以获取默认缩略图图像。简单代码应该是这样的：

//Grab the default thumbnail image
$attrs = $media->group->thumbnail[1]->attributes();
$thumbnail = $attrs['url'];
$thumbnail = substr($thumbnail, 0, -5);
$thumb1 = $thumbnail."default.jpg";

// Grab the third thumbnail image
$thumb2 = $thumbnail."2.jpg";

// Grab the fourth thumbnail image.
$thumb3 = $thumbnail."3.jpg";

// Using simple cURL to save it your server.
// You can extend the cURL below if you want it as fancy, just like
// the rest of the folks here.

$ch = curl_init ("$thumb1");
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_BINARYTRANSFER,1);
$rawdata = curl_exec($ch);
curl_close($ch);

// Using fwrite to save the above
$fp = fopen("SomeLocationInReferenceToYourScript/AnyNameYouWant.jpg", 'w');

// Write the file
fwrite($fp, $rawdata);

// And then close it.
fclose($fp);

您可以使用parse_url、parse_str从YouTube视频url获取视频ID，然后插入到图像的预测url中。感谢YouTube提供的预测URL

$videoUrl = "https://www.youtube.com/watch?v=8zy7wGbQgfw";
parse_str( parse_url( $videoUrl, PHP_URL_QUERY ), $my_array_of_vars );
$ytID = $my_array_of_vars['v']; //gets video ID

print "https://img.youtube.com/vi/$ytID/maxresdefault.jpg";
print "https://img.youtube.com/vi/$ytID/mqdefault.jpg";
print "https://img.youtube.com/vi/$ytID/hqdefault.jpg";
print "https://img.youtube.com/vi/$ytID/sddefault.jpg";
print "https://img.youtube.com/vi/$ytID/default.jpg";

您可以使用此工具生成YouTube缩略图

https://youtube-thumbnail-tool.com

虽然已经有很多答案，但对于新访客，我会留下两个获取缩略图的选项。

通过YouTube数据API获取缩略图

在Google Cloud Platform注册应用程序并激活YouTube Data API v3库在凭据部分创建API密钥。这样您将获得访问API的密钥并发送有关视频信息的请求，包括获取缩略图。

$api_key = 'YOUR_API_KEY';
$youtube_video_id = 'jNQXAC9IVRw';
$curl = curl_init();

curl_setopt_array($curl, array(
    CURLOPT_URL => 'https://www.googleapis.com/youtube/v3/videos?key='.$api_key.'&part=snippet&id='.$youtube_video_id,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_ENCODING => '',
    CURLOPT_MAXREDIRS => 10,
    CURLOPT_TIMEOUT => 0,
    CURLOPT_FOLLOWLOCATION => true,
    CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
    CURLOPT_CUSTOMREQUEST => 'GET',
)); 

$response = curl_exec($curl); 
curl_close($curl);

$response = json_decode($response,true); 

print_r($response); // result with video information and thumbnails

从没有API的直接链接获取缩略图

除了API之外，还可以通过直接链接获取缩略图，如下所示：

https://i.ytimg.com/vi/jNQXAC9IVRw/hqdefault.jpg

让我们详细考虑一下这个选项：

 https://i.ytimg.com/vi/<YOUTUBE_VIDEO_ID>/<SIZE_VALUE>.jpg 

哪里：

YOUTUBE_VIDEO_ID：您的视频IDSIZE_VALUE：缩略图大小。变量可以包含以下值：default、mqdefault、hqdefault、sddefault、maxresdefault

$youtube_video_id = 'jNQXAC9IVRw';
$size = 'hqdefault';

$curl = curl_init();
curl_setopt_array($curl, array(
    CURLOPT_URL => "https://i.ytimg.com/vi/{$youtube_video_id}/{$size}.jpg",
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_ENCODING => '',
    CURLOPT_MAXREDIRS => 10,
    CURLOPT_TIMEOUT => 0,
    CURLOPT_FOLLOWLOCATION => true,
    CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
    CURLOPT_CUSTOMREQUEST => 'GET',
));
$response = curl_exec($curl);
curl_close($curl);

// Write the file
$handle = fopen("image/filename.jpg", 'w'); // set your directory and filename
fwrite($handle, $response);
fclose($handle);

    function get_video_thumbnail( $src ) {
            $url_pieces = explode('/', $src);
            if( $url_pieces[2] == 'dai.ly'){
                $id = $url_pieces[3];
                $hash = json_decode(file_get_contents('https://api.dailymotion.com/video/'.$id.'?fields=thumbnail_large_url'), TRUE);
                $thumbnail = $hash['thumbnail_large_url'];
            }else if($url_pieces[2] == 'www.dailymotion.com'){
                $id = $url_pieces[4];
                $hash = json_decode(file_get_contents('https://api.dailymotion.com/video/'.$id.'?fields=thumbnail_large_url'), TRUE);
                $thumbnail = $hash['thumbnail_large_url'];
            }else if ( $url_pieces[2] == 'vimeo.com' ) { // If Vimeo
                $id = $url_pieces[3];
                $hash = unserialize(file_get_contents('http://vimeo.com/api/v2/video/' . $id . '.php'));
                $thumbnail = $hash[0]['thumbnail_large'];
            } elseif ( $url_pieces[2] == 'youtu.be' ) { // If Youtube
                $extract_id = explode('?', $url_pieces[3]);
                $id = $extract_id[0];
                $thumbnail = 'http://img.youtube.com/vi/' . $id . '/mqdefault.jpg';
            }else if ( $url_pieces[2] == 'player.vimeo.com' ) { // If Vimeo
                $id = $url_pieces[4];
                $hash = unserialize(file_get_contents('http://vimeo.com/api/v2/video/' . $id . '.php'));
                $thumbnail = $hash[0]['thumbnail_large'];
            } elseif ( $url_pieces[2] == 'www.youtube.com' ) { // If Youtube
                $extract_id = explode('=', $url_pieces[3]);
                $id = $extract_id[1];
                $thumbnail = 'http://img.youtube.com/vi/' . $id . '/mqdefault.jpg';
            } else{
                $thumbnail = tim_thumb_default_image('video-icon.png', null, 147, 252);
            }
            return $thumbnail;
        }

get_video_thumbnail('https://vimeo.com/154618727');
get_video_thumbnail('https://www.youtube.com/watch?v=SwU0I7_5Cmc');
get_video_thumbnail('https://youtu.be/pbzIfnekjtM');
get_video_thumbnail('http://www.dailymotion.com/video/x5thjyz');

在YouTube API V3中，我们还可以使用这些URL获取缩略图。。。它们是根据质量分类的。

https://i1.ytimg.com/vi/<insert-youtube-video-id-here>/default.jpg -   default
https://i1.ytimg.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg - medium 
https://i1.ytimg.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg - high
https://i1.ytimg.com/vi/<insert-youtube-video-id-here>/sddefault.jpg - standard

为了获得最大分辨率。。

https://i1.ytimg.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg

与第一个答案中的URL相比，这些URL的一个优点是这些URL不会被防火墙阻止。

我发现了一个很棒的工具，它可以让你用YouTube播放按钮创建图像：

安装在服务器上用于脚本编写：https://github.com/halgatewood/youtube-thumbnail-enhancer