Here is my solution (with key_exists function):
<?php
$text = readline();
$output = [];
for ($i = 0; $i < strlen($text); $i++) {
$char = $text[$i];
if (!key_exists($char, $output)) {
$output[$char] = 0;
}
$output[$char]++;
}
foreach ($output as $k => $v) {
echo "$k -> $v" . PHP_EOL;
}