compress.php

<?php

function compress(array $input) {
    $result = [];
    while (isset($input[0])) {
        $i = 0;
        do {
            ++$i;
            $letter = array_shift($input);
        } while (isset($input[0]) && $letter === $input[0]);
        array_push($result, $letter, "{$i}");
    }
    return $result;
}

var_dump(compress(["a", "a", "b", "b", "b", "c", "c", "d"]));
/**
array(8) {
[0]=> string(1) "a"
[1]=> string(1) "2"
[2]=> string(1) "b"
[3]=> string(1) "3"
[4]=> string(1) "c"
[5]=> string(1) "2"
[6]=> string(1) "d"
[7]=> string(1) "1"
}
 */