<?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;

?>

Elvisten10: ^^^, i guess i did not read the question well and missed the i need your algo part. Here is my code for this:
<?php
function mixString($str1, $str2) {
$str = $str1.$str2;
$i = strlen($str);
$temp = '';
while($i > 1) {
$i = $i - 1;
$j = mt_rand(0, $i);
$temp = $str[$j];
$str[$j] = $str[$i];
$str[$i] = $temp;
}
return $str;
}
echo mixString('Elvis', 'Chidera');
echo "\n";
echo mixString('Elvis', '');
?>

http://ideone.com/MzyBlV

Kidstell: the arrangement is definite and specific not random but i can notice your legend grade coding. My solution is not the best but i'm still reserving it

// $a and $b already known
$a='temple';$b='agile';
$al=strlen($a); $bl=strlen($b);
 $diff=$bl-$al; $offset=abs($diff);
if($diff>0){
 $twil=$b; $lite=$a;
}else{
$twil=$a;$lite=$b;
}
$tl=strlen($twil); $l=strlen($lite));
 ($tl>$l)?$part1=substr($twil,0,$offset-1):$part1='';
 ($tl>$l)?$mix1=substr($twil,$offset-1,$tl-1):$mix1=$twil;
$mix1=explode($mix1,''); $mix2=explode($lite,''); $mix='';
for($i=0;$i<$l;$i++){
 $mix.=$mix1[$i].$mix2[$i];
}
$answer=$part1.$mix;
echo "string1= $a"; echo "string2= $b";
echo $answer;

Javanian: whats with PHP and $$$$$$$$$$$$$ it makes the code look ugly

Elvisten10: @kidstell, some errors i fixed in that code:
1)explode, you should have used str_split
2)double )) in your code.

Here is the edited copy www.ideone.com/KmIa6G

Javanian:
whats with PHP and $$$$$$$$$$$$$ it makes the code look ugly