<?php

$imageIndex = isset($_GET['p']) ? intval($_GET['p']) : 0;

$directory = 'test2';

$iterator = new RecursiveIteratorIterator( 
											new RecursiveDirectoryIterator($directory,RecursiveDirectoryIterator::SKIP_DOTS),
											RecursiveIteratorIterator::LEAVES_ONLY);

$ext = array("png", "jpg", "jpeg");

foreach ($iterator as $fileinfo) {

    if (in_array($fileinfo->getExtension(), $ext)) {
        $images[] = $fileinfo->getFileName();
    }
	
}

if(!isset($images[$imageIndex])) $imageIndex = 0;

$nextImage = isset($imageIndex) ? $imageIndex + 1 : 1;
$prevImage = isset($imageIndex) && ($imageIndex != 0) ? $imageIndex - 1 : 0;

?>
<img src="<?=$directory.'/'.$images[$imageIndex]?>">
<br>
<a href="<?=$_SERVER['PHP_SELF']?>?p=<?=$prevImage?>">geri</a>

<a href="<?=$_SERVER['PHP_SELF']?>?p=<?=$nextImage?>">ileri</a>