<?php
function isGIF($filename)
{
    $filecontents = file_get_contents($filename);
    $str_loc = 0;
    $count = 0;
    while ($count < 2) {
        $where1 = strpos($filecontents, "\x00\x21\xF9\x04", $str_loc);
        if ($where1 === false) {
            break;
        } else {
            $str_loc = $where1+1;
            $where2 = strpos($filecontents, "\x00\x2C", $str_loc);
            if ($where2 === false) {
                break;
            } else {
                if ($where1+8 == $where2) {
                    $count++;
                }
                $str_loc = $where2+1;
            }
        }
    }
    if ($count > 1) {
        return(true);
    } else {
        return(false);
    }
}