js代码:视频文件和js代码最好为同一服务器下，不然容易出错

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
</head>
<body>
    <video id="video" controls="controls" width="400" height="300" preload="auto">
        <source src="./upload/63534e2688f81.mp4">
    </video>
    <br>
    <br> 视频第一帧图片：
    <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);
            };
            var captureImage = function() {
                var canvas = document.createElement("canvas");
                canvas.width = video.videoWidth * scale;
                canvas.height = video.videoHeight * scale;
                canvas.getContext('2d').drawImage(video, 0, 0, canvas.width,canvas.height);
                var img = document.createElement("img");
                img.src = canvas.toDataURL("image/png");
                img.width = 400;
                img.height = 300;
                console.log(img);
                output.appendChild(img);
            };
            initialize();
        })();
    </script>
</body>
</html>

php代码：将js获取到的base64数据转为文件

file_put_contents('./aa.jpg',base64_decode(explode(',',$base64)[1]));

js 转视频第一帧为base64图片，用php转为图片文件

js代码:视频文件和js代码最好为同一服务器下，不然容易出错

php代码：将js获取到的base64数据转为文件