<?php

$string = "akkaka ka";

print_stats(get_letter_stats($string, 1));
print "-------\n";
print_stats(get_letter_stats($string, 2));

function print_stats($stats) {
  $sum = 0;
  foreach($stats as $v) {
    $sum += $v;
  }
  foreach($stats as $k=>$v) {
    print "'".$k."': ".$v."(".round(100*$v/$sum)."%)\n";
  }
}

function get_letter_stats($string, $length) {
  $stats = array();
  for($i=0; $i<strlen($string); $i++) {
    $tmp = substr($string, $i, $length);
    if (strlen($tmp) == $length)
      $stats[$tmp]++;
  }
  arsort($stats);
  return $stats;
}