[RESOLVED] Directories not being detected

[noahssite]

Hello, I wrote a simple script to display all the files/folders in a directory. When you click a file it will open the file. When you click a directory it SHOULD list all the files/folders in that directory.

Here is my script:

<?php
$html="";
$dir='.';
if(isset($_GET['dir'])){
if(strlen($_GET['dir']) > 0){
$dir = $_GET['dir'];
}
}
if ($handle = opendir($dir)) {
    $html .= "<table style='border: 1px solid black;'>";
    while (false !== ($file = readdir($handle))) {
        if ($file != "." && $file != "..") {
        if(!is_dir($file)){
         $html .= "<tr><td><a href='".$dir.'/'.$file."'>$file</a></td></tr>";
        }
        else{
        $html .= "<tr><td><a href='index.php?dir=".$dir.'/'.$file."'>$file</a></td></tr>";
        }
    }
    }
    $html .= "</table>";
    closedir($handle);
}
echo $html;
?>

Don't just correct the code, please provide an explanation so I don't repeat this error in the future.