刷新就改变背景图片是怎么做的?

<?php

$device = $_GET['device'];//设备

if($device == null || $device == 'auto' || $device == 'AUTO'){
    if(wp_is_mobile() == true){
        $img_array = glob("Store/Pe/*.{gif,jpg,png}",GLOB_BRACE); 
    }else{
        $img_array = glob("Store/Pc/*.{gif,jpg,png}",GLOB_BRACE); 
    }
}else{
    if($device == 'pc' || $device == 'Pc' || $device == 'PC'){
         $img_array = glob("Store/Pc/*.{gif,jpg,png}",GLOB_BRACE); 
    }else if($device == 'pe' || $device == 'Pe' || $device == 'PE'){
         $img_array = glob("Store/Pe/*.{gif,jpg,png}",GLOB_BRACE); 
    }else{
         $img_array = glob("Store/Pc/*.{gif,jpg,png}",GLOB_BRACE);
    }

}

$img = array_rand($img_array); 
header("Location:".$img_array[$img]);

//判断手机或者电脑
function wp_is_mobile() {
 static $is_mobile = null;

 if ( isset( $is_mobile ) ) {
  return $is_mobile;
 }

 if ( empty($_SERVER['HTTP_USER_AGENT']) ) {
  $is_mobile = false;
 } elseif ( strpos($_SERVER['HTTP_USER_AGENT'], 'Mobile') !== false // many mobile devices (all iPhone, iPad, etc.)
  || strpos($_SERVER['HTTP_USER_AGENT'], 'Android') !== false
  || strpos($_SERVER['HTTP_USER_AGENT'], 'Silk/') !== false
  || strpos($_SERVER['HTTP_USER_AGENT'], 'Kindle') !== false
  || strpos($_SERVER['HTTP_USER_AGENT'], 'BlackBerry') !== false
  || strpos($_SERVER['HTTP_USER_AGENT'], 'Opera Mini') !== false
  || strpos($_SERVER['HTTP_USER_AGENT'], 'Opera Mobi') !== false ) {
   $is_mobile = true;
 } else {
  $is_mobile = false;
 }

 return $is_mobile;
}
?>