restricting/securing filebrowser in php

I've built my filebrowser so far but it's pretty insecure. For example when you insert "?dir=../" or "?dir=../../../" you can view files you shouldn't be able to. My code:

<?php
    if(isset($_GET['dir']) && $_GET['dir'] !== "../") {$dir = $basedir.$_GET['dir']."/";} else {$dir = $basedir;}
    if ($handle = opendir($dir)) {
        if(isset($_GET['dir']) && $_GET['dir'] !== "") {echo "<a href='?dir='>back to root</a>";}
        while (false !== ($file = readdir($handle))) {
            if ($file != "." && $file != "..") {
                if(filetype($dir . $file) == "file") {$filetype = what_suffix($file);} else {$filetype = "-";}
                if(filetype($dir . $file) == "file") {$filesize = round( filesize($dir . $file)/1024/1024 ,2)." MB";} else {$filesize = "-";}
                if(filetype($dir . $file) == "file") {$filedate = date("d.m.Y H:i:s", filectime($dir.$file));} else {$filedate = "-";}
                if(filetype($dir . $file) == "file") {
                echo(
                    "<tr>
                        <td><a href='?dir=".@$_GET['dir']."&file=".$file."'>".$file."</td>
                        <td>".$filetype."</td>
                        <td>".$filesize."</td>
                        <td>".$filedate."</td>
                    </tr>"
                ); } else {
                echo(
                    "<tr>
                        <td><a href='?dir=".@$_GET['dir']."/".$file."'>".$file."</td>
                        <td>-</td>
                        <td>-</td>
                        <td>-</td>
                    </tr>"
                ); }


            }
        }
        closedir($handle);
    }
?>

Is there a way I could restrict that? (I guess I just don't see the wood for the trees)

Solution

Here is a basic security check I have used for such things.

function isSecurityViolation($dir) {
    if (preg_match('#https?://#i', $dir)) {
        return true;
    }

    // no directory traversal attempts
    if (strpos($dir, '../') !== false) {
        return true;
    }

    if (strpos($dir, "\x00") !== false) {
        return true;
    }

    if (strpos($dir, '%00') !== false) {
        return true;
    }

    return false;
}

It checks for the following violations:

Attempting to open an http or https URL
Directory traversal using ..
Null character checks which can allow malicious urls to pass certain functions

Somewhere towards the beginning of your script, you could call:

if (isSecurityViolation($dir)) {
    die('No soup for you.');
}