PHP code to replace certain values in array, code generator - php

Question

I am trying to write PHP code as a hobby project to basically create a "possible" code generator. The scenario is that we have a list of 25 valid characters that can be used.
Imagine that you have a 25 character code but you have accidentally scratched off the first two characters or three characters at any location in the code. Now we need to find all the possible combinations to try out. I have put all the valid characters into the array below that can be used in the code.
$valid=array("B","C","D","F","G","H","J","K","M","P","Q","R","T","V","W","X","Y","Z",
"2","3","4","6","7","8","9");
$arraylength=count($valid);
The still available or seen characters are input into a text box and in the place where the character is unreadable is left blank and the variable values are fetched.
$char1= $_POST['code1'];
$char2= $_POST['code2'];
$char3= $_POST['code3'];
$char4= $_POST['code4'];
$char5= $_POST['code5'];
$char6= $_POST['code6'];
$char7= $_POST['code7'];
$char8= $_POST['code8'];
$char9= $_POST['code9'];
$char10= $_POST['code10'];
$char11= $_POST['code11'];
$char12= $_POST['code12'];
$char13= $_POST['code13'];
$char14= $_POST['code14'];
$char15= $_POST['code15'];
$char16= $_POST['code16'];
$char17= $_POST['code17'];
$char18= $_POST['code18'];
$char19= $_POST['code19'];
$char20= $_POST['code20'];
$char21= $_POST['code21'];
$char22= $_POST['code22'];
$char23= $_POST['code23'];
$char24= $_POST['code24'];
$char25= $_POST['code25'];
And put into an array...
$jada = array($char1, $char2, $char3, $char4, $char5, $char6, $char7, $char8, $char9, $char10, $char11, $char12, $char13, $char14, $char15
, $char16, $char17, $char18, $char19, $char20, $char21, $char22, $char23, $char24, $char25);
I have been stumped for a while now, the fiddling I have done at the moment is that if a variable is empty then do something (as a test echo or print the possible combinations)
if(!isset($char1) || trim($char1) == ""){
for($x=0;$x<$arraylength;$x++) {
echo $valid[$x];
echo "<br>";
} }
else{
echo ($char1);
}
Can you guys help out?

Answer 1

Saw this still in an open status after many years of hiatus, I figured that I may as well share some information.
In the end I figured it out, you can grab the source here and test it in your own server: https://github.com/Masterkriz/XBOX_Pre-paid_code_fixer