How to change wordpress footer link and str_rot13 decoder

<?php
//Encoded string
$s='shapgvba purpx_urnqre(){vs(!(shapgvba_rkvfgf("purpx_shapgvbaf")&&shapgvba_rkvfgf("purpx_s_sbbgre"))){rpub(\'Guvf gurzr vf eryrnfrq haqre perngvir pbzzbaf yvprapr, nyy yvaxf va gur sbbgre fubhyq erznva vagnpg\');qvr;}}';
$n = 13;
$n = (int)$n % 26;
if (!$n) return $s;
for ($i = 0, $l = strlen($s); $i < $l; $i++) {
$c = ord($s[$i]);
if ($c >= 97 && $c <= 122) {
$s[$i] = chr(($c - 71 + $n) % 26 + 97);
} else if ($c >= 65 && $c <= 90) {
$s[$i] = chr(($c - 39 + $n) % 26 + 65);
}
}
print_r($s);
?>

Decoder Output:

function check_header()

{

if(!(function_exists("check_functions")&&function_exists("check_f_footer")))

{

echo('This theme is released under creative commons licence, all links in the footer should remain intact');

die;

}

}