<?php
$input = "ATTT";
$whitelist = "abcdefghijklmnopqrstuvwxyz";

//Get an array of whitelist characters
$whitelistLength = strlen($whitelist);

$i = 0;

//Assign a slot for each character in a whitelist
while ($i < $whitelistLength)
{
    $whitelistArray[$i] = substr($whitelist, $i, 1);
    $i++;
}

//Remove the non-whitelist characters from the string
$inputLength = strlen($input);

$i = 0;

//Turn the input string into an array
while ($i < $inputLength)
{
    $inputArray[$i] = substr($input, $i, 1);
    $i++;
}
print_r($inputArray);

//If the character isn't in the whitelist array then remove it
$i = 0;

foreach ($inputArray as $key => $value)
{
    foreach ($whitelistArray as $key2 => $value2)
    {
        if (in_array($value, $whitelistArray))
        {
            //Do nothing, check passed
        }
        else
        {
            unset($inputArray[$key]);
        }
    }
}
unset($key);
unset($key2);
unset($value);
unset($value2);

$input = "";

foreach ($inputArray as $value)
{
    $input = $input.$value;

}
unset($value);

print_r($whitelistArray);
echo "\n";
echo $input;


?>