freq_seq.php

<?php
// Return an output string that translates an input string s/$s by replacing each character in s/$s with a number representing the number of times that character occurs in s/$s and separating each number with the character(s) sep/$sep.
//
// freq_seq("hello world", "-"); // => "1-1-3-3-2-1-1-2-1-3-1"
// freq_seq("19999999", ":"); // => "1:7:7:7:7:7:7:7"
// freq_seq("^^^**$", "x"); // => "3x3x3x2x2x1"

function freq_seq(string $s, string $sep): string {
  $result = array_map(function($x) use ($s) {
    return substr_count($s, $x);
  }, str_split($s));
  return join($sep, $result);
}

// Alternative solution:
// function freq_seq(string $s, string $sep): string {
//     $results = [];
//
//     $chars = str_split($s);
//     $countValues = array_count_values($chars);
//     foreach ($chars as $char) {
//         $results[] = $countValues[$char];
//     }
//
//     return implode($sep, $results);
// }

// function freq_seq(string $s, string $sep): string {
//   $s = str_split($s);
//   $dict = array_count_values($s);
//   return join($sep, array_map(function ($char) use ($dict) {
//     return $dict[$char];
//   }, $s));
// }


?>