[PHP] "Verschlüsselung" läuft schief :(

<?php
 if (isset($_POST['ok']))
 {
 $str_alph = $_POST['textbox'];
 $str_ralph = qrstufdpcukhumglyjzbeoiawx;
 
 for ($i = 0; $i < strlen($str_alph); $i++)
 {
 $arr_alph[] = substr ($str_alph, $i, 1);
 }

 for ($i = 0; $i < strlen($str_ralph); $i++)
 {
 $arr_ralph[] = substr ($str_ralph, $i, 1);
 }

 $posa=0;
 $posr=0;
 
 while ($posa<count($arr_alph)){

 if ($posr<=count($arr_ralph))
 {
 $bsa=$arr_alph[$posa];
 $bsr=$arr_ralph[$posr];
 $bsa=$bsr;
 $gt=$gt.$bsa;
 $posr++;
 } 
 else
 $posr=o;

 $posa++;
 }

 echo $gt;
 echo "<br>";
 }

 echo "<form action='rotorkomplex.php' method='POST'>
 <input type='text' name='textbox'>
 <input type='submit' name='ok' value='Und los...'>";
?>