function createVideoThumb($fileName,$dir,$file = null){
 
    $movie = new ffmpeg_movie($dir.'/'.$fileName);//这里就是视频的存储路径
    $ff_frame = $movie->getFrame(1);
    $gd_image = $ff_frame->toGDImage();
 
    $filenames =substr($fileName,0,strrpos($fileName,'.')); 
    $img="./upload/".$filenames."_video.jpg";//存图片的路径
    imagejpeg($gd_image, $img);
    imagedestroy($gd_image);
 
}

    <video id="video" controls="controls">
        <source src="123.mp4">
    </video>
    <div id="output"></div>
    <script type="text/javascript">
    (function() {
        var video, output;
        var scale = 0.8;
        var initialize = function() {
            output = document.getElementById("output");
            video = document.getElementById("video");
            video.addEventListener('loadeddata', captureImage); // 用于向指定元素添加事件句柄。
        };

        var captureImage = function() {
            var canvas = document.createElement("canvas"); // 创建一个画布
            canvas.width = video.videoWidth * scale;
            canvas.height = video.videoHeight * scale;
            canvas.getContext('2d').drawImage(video, 0, 0, canvas.width, canvas.height); // getContext:设置画布环境；drawImage:画画 

            var img = document.createElement("img");
            img.src = canvas.toDataURL("image/png"); // 获取图片的url
            output.appendChild(img);
        };

        initialize();
    })();
    </script>