Count repeated chars sequence and build a string with its chars (only 1) + repeated numbers

count_chars.php

<?php
$input  = 'aaacccbdd';
$output = 'a3b1c3d2';

$list = str_split($input);

$matchs = [];

foreach ($list as $i => $char) {
    isset($matchs[$char]) ? $matchs[$char]++ : $matchs[$char] = 1;
}

ksort($matchs);

$builded = '';

foreach ($matchs as $i => $num) {
    $builded .= $i.$num;
}

var_dump($builded);
var_dump($builded === $output);