PHP random Facebook Cover - Tile Algorithm

<?php

function getfiles($dir){
    $return = array('files' => array(), 'count' => 0);
    $num = 0;
    $allowed_extension = array('jpeg', 'jpg', 'png', 'gif');
    if(is_dir($dir)){
        $files = scandir($dir);
        foreach($files as $file){
            $file = $dir.DIRECTORY_SEPARATOR.$file;
            if(is_file($file)){
                $extension = pathinfo($file, PATHINFO_EXTENSION);
                $extension = strtolower($extension);
                if(in_array($extension, $allowed_extension, true)){
                    $return['files'][] = array('path' => realpath($file), 'extension' => $extension);
                    $return['count']++;
                }
            }
        }
    }
    return $return;
}

$height = 315;
$width = 850;

// Fb covers are 850 x 315

$images = getfiles('img'); // scan img folder for tile images
shuffle($images['files']);
set_time_limit(0);

$columns = 4;
$px = 0;
$py = 0;
$new_width = 0;
$new_height = 0;
$column_count = 0;
$vary = floor($width/10);

$image = imagecreatetruecolor($width, $height);
imagefilledrectangle($image, 0, 0, $width, $height, imagecolorallocate($image, 255, 255, 255));

for($i = 0; $i < $images['count']; $i++){
    if($images['files'][$i]['extension'] == 'jpg' || $images['files'][$i]['extension'] == 'jpeg'){
        $tmp_image = imagecreatefromjpeg($images['files'][$i]['path']);
    }else if($images['files'][$i]['extension'] == 'png'){
        $tmp_image = imagecreatefrompng($images['files'][$i]['path']);
    }else if($images['files'][$i]['extension'] == 'gif'){
        $tmp_image = imagecreatefromgif($images['files'][$i]['path']);
    }
    $tmp_width = imagesx($tmp_image);
    $tmp_height = imagesy($tmp_image);
    if($py == 0){
        $new_width = rand(ceil($width/$columns) - $vary, ceil($width/$columns) + $vary);
        $column_count++;
    }
    if($column_count == $columns){
        $new_width = $width - $px;
    }
    $new_height = floor($new_width * ($tmp_height/$tmp_width));

    imagecopyresampled($image, $tmp_image, $px, $py, 0, 0, $new_width, $new_height, $tmp_width, $tmp_height);
    $py += $new_height;
    if($py >= $height){
        $py = 0;
        $px += $new_width;
    }
    //echo $px.','.$py.'<br />';
    imagedestroy($tmp_image);
}

header('Content-type: image/png');
imagepng($image);
imagepng($image, 'cover.png', 9);
imagedestroy($image);
?>