Here is my solution:
<?php
$string = "";
$input = readline();
while ($input != "End") {
$args = explode(" ", $input);
$command = $args[0];
switch ($command) {
case 'Print':
echo $string . PHP_EOL;
break;
case 'Add':
$concat = $args[1];
$string .= $concat;
break;
case 'Upgrade':
$char = $args[1];
$replace = chr(ord($char) + 1);
$string = str_replace($char, $replace, $string);
break;
case 'Index':
$indeces = [];
$search = $args[1];
for ($i = 0; $i < strlen($string); $i++) {
if ($string[$i] == $search) {
$indeces[] = $i;
}
}
if (count($indeces) > 0) {
echo implode(" ", $indeces) . PHP_EOL;
} else {
echo "None";
}
break;
case 'Remove':
$remove = $args[1];
$string = str_replace($remove, "", $string);
break;
}
$input = readline();
}