<?php
/**
 * @author adewasco2k
 * @copyright 2013
 */
$a= "$`_POST['string1']"; 
$b= "$`_GET['key']"; 
//$answer="$_POST$[_'GsEtTr[i'nkge1y'']]";
$answer=""; //empty string to hold our answer
$x=0;
$strtest= strlen($a)>= strlen($b) ? $a : $b; //check which is longer in lenght
for($i=0;$i<strlen($strtest); $i++)
{
    if($i<=6)   // this is to get the first $_POST
    {
        $answer.=$a[$i];
    }else     // now we got $_POST lets mix the string 
    {
     $answer.=$b[$x];
     $answer.=$a[$i];
     $x++;
    }
}
echo $answer;
?>

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