Display random image from a directory in a standard img src using php

<?php
 
// set the $folder to the folder containing the images
// You then use <img src="/path/to/this/script.php" alt="image randomiser" />
// to display them
 
  $folder = 'banners';
 
  $extList = array();
	$extList['gif'] = 'image/gif';
	$extList['jpg'] = 'image/jpeg';
	$extList['jpeg'] = 'image/jpeg';
	$extList['png'] = 'image/png';
 
 
  $img = null;
 
  if (substr($folder,-1) != '/') {
  	$folder = $folder.'/';
  }
 
  if (isset($_GET['img'])) {
  	$imageInfo = pathinfo($_GET['img']);
  	if (
  	    isset( $extList[ strtolower( $imageInfo['extension'] ) ] ) &&
          file_exists( $folder.$imageInfo['basename'] )
      ) {
		$img = $folder.$imageInfo['basename'];
	}
  } else {
  	$fileList = array();
  	$handle = opendir($folder);
  	while ( false !== ( $file = readdir($handle) ) ) {
  		$file_info = pathinfo($file);
  		if (
  		    isset( $extList[ strtolower( $file_info['extension'] ) ] )
  		) {
  			$fileList[] = $file;
  		}
  	}
  	closedir($handle);
 
  	if (count($fileList) > 0) {
  		$imageNumber = time() % count($fileList);
  		$img = $folder.$fileList[$imageNumber];
  	}
  }
 
  if ($img!=null) {
  	$imageInfo = pathinfo($img);
  	$contentType = 'Content-type: '.$extList[ $imageInfo['extension'] ];
  	header ($contentType);
  	readfile($img);
  } else {
  	if ( function_exists('imagecreate') ) {
  		header ("Content-type: image/png");
  		$im = @imagecreate (100, 100)
  		    or die ("Cannot initialize new GD image stream");
  		$background_color = imagecolorallocate ($im, 255, 255, 255);
  		$text_color = imagecolorallocate ($im, 0,0,0);
  		imagestring ($im, 2, 5, 5,  "IMAGE ERROR", $text_color);
  		imagepng ($im);
  		imagedestroy($im);
  	}
  }
 
 
 
?>