php如何对字节流bytes进行加密解密呢

php如何对字节流bytes进行加密解密呢？

如下面这段代码，打开文件后输出字节流，那么这个字节流怎么加密和解密？

<?php
    $file_name = "demo.mp4";
    $file_sub_path = "../video/";
    $file_path = $file_sub_path.$file_name;
    $fp = fopen($file_path, "r");
    $file_size = filesize($file_path);
    //下载文件需要用到的头
    Header("Content-type: application/octet-stream");
    Header("Accept-Ranges: bytes");
    Header("Accept-Length:".$file_size);
    Header("Content-Length:".$file_size);
    Header("Content-Disposition: attachment; filename=".$file_name);

    $buffer = 1024;
    $file_count = 0;
    while (!feof($fp) && $file_count < $file_size) {
        $file_con = fread($fp, $buffer);
        $file_count += $buffer;
        echo $file_con;//这个数据怎么加密
    }
    fclose($fp);
?>