Polybius cipher decryption

I am doing my school homework, its polybius square cipher. I have already done encryption off input string, but i dont know how to decrypt output string back to input string (aka how to decrypt).

---

First, i include main php file:

<?php


for ($i=1; $i<=7; $i++)
        {
        for ($j=1; $j<=7; $j++)
                {
                if (chr($j+7*($i-1)+47))
                        $coords[chr($j+7*($i-1)+47)]=$i.$j;
                else
                        $coords[chr($j+7*($i-1)+47)]=$i.$j;
                }
        }


$input=str_split(strtoupper($text));
$c=count($input);
$output='';
for ($i=0; $i<$c;$i++)
        if ($input[$i]!=' ')
                $output.=$coords[$input[$i]];
        else
                $output.=' ';
$search  = array('1', '2', '3', '4', '5', '6', '7');
$replace = array('A', 'B', 'C', 'D', 'E', 'F', 'G');
$output = str_replace($search, $replace, $output);

?>

to this html file:

<form action="?page=polybius" method="post">
        <textarea name="text"><?= $_POST[$text]; ?><?php echo $output;?></textarea><br/>
<input type="submit" name=cipher" value=" cipher "/><br><br>
</form>

---

I would like to decode $output back to $input

• encryption: A = $coords of A in polybius square


• decryption: $coords of A = to A

(like this simple caesar cipher code: http://ift.tt/1CrWiVe

Thank you for any suggestions.