<?php

$replaceTable = [ 
    'A' => 'Z', 
    'B' => 'Y', 
    'C' => 'X', 
    'D' => 'W',
    'E' => 'V', 
    'F' => 'O', 
    'G' => 'P', 
    'H' => 'Q',
    'I' => 'R', 
    'J' => 'S', 
    'K' => 'T', 
    'L' => 'U',
    'M' => 'N',
    'N' => 'M', 
    'O' => 'L', 
    'P' => 'K', 
    'Q' => 'J',
    'R' => 'I', 
    'S' => 'H', 
    'T' => 'G', 
    'U' => 'F',
    'V' => 'E', 
    'W' => 'D', 
    'X' => 'C', 
    'Y' => 'B',
    'Z' => 'A'
];

$string = "HelloWorld";

echo strtr(strtoupper($string), $replaceTable);