PHP encode/decode

Question

I had this code:

eval(str_rot13('shapgvba purpx_sbbgre(){tybony $nz_bcgvba;$y=\'<n uers="uggc://jjj.penmlkugzy.pbz" gnetrg="_oynax">CFQ gb UGZY PFF</n>\';$s=GRZCYNGRCNGU.\'/sbbgre.cuc\';$sq=sbcra($s,\'e\');$p=sernq($sq,svyrfvmr($s));spybfr($sq);vs(fgecbf($p,$y)==0){rpub($nz_bcgvba[\'phfgbz\'][\'cevinpl\']);qvr;}}purpx_sbbgre();'));

I used to decode it with http://www.unphp.net/

Which became this code:

<?php function check_footer() {
    global $am_option;
    $l = '<a href="http://www.crazyxhtml.com" target="_blank">PSD to HTML CSS</a>';
    $f = TEMPLATEPATH . '/footer.php';
    $fd = fopen($f, 'r');
    $c = fread($fd, filesize($f));
    fclose($fd);
    if (strpos($c, $l) == 0) {
        echo ($am_option['custom']['privacy']);
        die;
    }
}
function check_footer() {
    global $am_option;
    $l = '<a href="http://www.crazyxhtml.com" target="_blank">PSD to HTML CSS</a>';
    $f = TEMPLATEPATH . '/footer.php';
    $fd = fopen($f, 'r');
    $c = fread($fd, filesize($f));
    fclose($fd);
    if (strpos($c, $l) == 0) {
        echo ($am_option['custom']['privacy']);
        die;
    }
}
check_footer();

Can someone help me how can I encode this back? I mean I changed somethings but dont know how to encode in similar language/code.

Thank you in advance.

Answer 1

Rot13 just shifts ascii letters 13 characters. Since there are 26 letters in the ASCII alphabet, this means shifting them 13 characters again will reverse the operation. Since non-ascii-letters will be ignored by the operation altogether, the reverse of rot13 is just rot13 again, as you can see below:

> $x='shapgvba purpx_sbbgre(){tybony $nz_bcgvba;$y=\'<n uers="uggc://jjj.penmlkugzy.pbz" gnetrg="_oynax">CFQ gb UGZY PFF</n>\';$s=GRZCYNGRCNGU.\'/sbbgre.cuc\';$sq=sbcra($s,\'e\');$p=sernq($sq,svyrfvmr($s));spybfr($sq);vs(fgecbf($p,$y)==0){rpub($nz_bcgvba[\'phfgbz\'][\'cevinpl\']);qvr;}}purpx_sbbgre();';
> var_dump($x === str_rot13(str_rot13($x)));
bool(true)