get a frame out of a video

framegrapper.php

<?php
    header("Content-type: image/png");
    $url = $_GET["url"]; 
    $pos =  $_GET["pos"]; 
    $hashtag = md5($url);
    $transcoder_info = shell_exec("./transcoder.sh  -vframes 1 -ss $pos -i $url /tmp/$hashtag.png 2>&1"); 
    $data = file_get_contents("/tmp/$hashtag.png");
    header("Content-type: image/png");
    echo $data;
?>

transcoder.sh

#!/bin/sh
/opt/local/bin/ffmpeg $@