Metni Basitçe Şifreleyerek Dosya Oluşturmak

<?php
header('Content-Type: text/html; charset=utf-8');
mb_internal_encoding('UTF-8');
class Sifre{
    public $bundan=['1','2','3','4','5','6','7','8','9','0',
    'a','b','c','ç','d','e','f','g','ğ','h','ı','i','j','k','l',
    'm','n','o','ö','p','r','s','ş','t','u','ü','v','w','x','y','z',
    'A','B','C','Ç','D','E','F','G','Ğ','H','I','İ','J','K','L',
    'M','N','O','Ö','P','R','S','Ş','T','U','Ü','V','W','X','Y','Z'];
    public $buna=['v','B','i','Y','V','T','K','0','İ','7','n',
    'Ç','x','ğ','b','f','o','Ü','J','z','Ğ','u','9','O','r','W',
    'ş','A','g','ü','4','P','R','5','6','Z','w','s','E','3','y','F',
    'd','I','D','t','ç','L','8','m','C','e','ö','ı','a','1','G','H',
    'M','l','p','2','X','h','j','k','Ö','U','N','Ş','S','c'];
    function mb_chars_replace($from, $to, $subj, $delSymb='_') {
     $nsubj='';
     preg_match_all('/(.)/u', $subj, $subj);$subj=$subj[1];
     if (!is_array($from)) {preg_match_all('/(.)/u', $from, $from);$from=$from[1];} 
     if (!is_array($to)) {preg_match_all('/(.)/u', $to, $to);$to=$to[1];}
     if (count($from)!==count($to)) return false;
     foreach($subj as $s) {
       foreach($from as $k=>$f) {
         if($s===$f) {
                    $s=$to[$k];
                    break;
         }
       }
       if($s!==$delSymb) $nsubj.=$s;
     }
    return $nsubj;
    } 
    function Sifrele($metin,$dosya){
        $metin=$this->mb_chars_replace($this->bundan,$this->buna,$metin);
        file_put_contents($dosya,$metin);
        return $metin;
    }
    function Coz($dosya) {
        return $this->mb_chars_replace($this->buna,$this->bundan, file_get_contents($dosya));
    }
}
$foo = new Sifre;
echo $foo->Sifrele('Merhaba dünya, bu şifreli bir metin!','test.txt');
echo "<br>";
echo $foo->Coz('test.txt');
?>
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Gf4znÇn bZş3n, Ç6 Ruo4fru Çu4 Wf5uş!
Merhaba dünya, bu şifreli bir metin!
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