Generate a code by replacing user input based on an associative array

Question

I have a 16-character, input string composed of hexadecimal characters. I need to isolate the last two characters, then replace the first character using an associative lookup/map, then extend the string from two to four characters by repeating the characters.

My sample input is:

$atcode = '11F5796B61690196';

My associative array is:

$sub1 = [
    '0' => '2',
    '1' => '3',
    '2' => '0',
    '3' => '1',
    '4' => '6',
    '5' => '7',
    '7' => '5',
    '8' => 'A',
    '9' => 'B',
    'A' => '8',
    'B' => '9',
    'C' => 'E',
    'D' => 'F',
    'E' => 'C',
    'F' => 'D'
];

The isolated characterd should be 9 and 6.

The desired result is: B6B6

This is the code so far:

$codearray = str_split($atcode);
    
//select the 15th digit from $codearray and name it $one
$one = $codearray[14];
    
//create an associative array and name it $sub1
$sub1 = array('0' => '2', '1' => '3', '3' => '1', 'D' => 'F', '4' => '6', '5' => '7', '7' => '5', '8' => 'A', '9' => 'B', 'A' => '8', 'B' => '9', 'C' => 'E', '2' => '0', 'E' => 'C', 'F' => 'D');
    
//search through $sub1, determine which value is equivalent to $one, and select its equivalency from the key to value pairs
$codeone = array_replace($one, $sub1);
    
//select the 16th digit from $codearray and name it $two
$two = $codearray[15];
    
echo "$codeone", "$two"; 

However, I am just getting a blank page.

Answer 1

Abandon most of that code.

Extract the last two characters, translate the first, then repeat them.

Code: (Demo)

var_export(
    str_repeat(
        strtr(
            $atcode[14],
            '012345789ABCDEF',
            '2301675AB89EFCD'
        ) . $atcode[15],
        2
    )
);

Or if you want to use the associative array: Demo

var_export(
    str_repeat(
        strtr($atcode[14], $sub1) . $atcode[15],
        2
    )
);

Here's an alternative, for the regex lovers: Demo

var_export(
    preg_replace_callback(
        '/.*(.)(.)/',
        fn($m) => str_repeat(strtr($m[1], $sub1) . $m[2], 2),
        $atcode
    )
);