count the occurrences of all the letters in a string PHP

There is a php function that returns information about characters used in a string: count_chars

Well it might not be what you are looking for, because according to http://php.net/manual/en/function.count-chars.php it

Counts the number of occurrences of every byte-value (0..255) in string and returns it in various ways

Example from same link (http://php.net/manual/en/function.count-chars.php):

<?php
$data = "Two Ts and one F.";

foreach (count_chars($data, 1) as $i => $val) {
   echo "There were $val instance(s) of \"" , chr($i) , "\" in the string.\n";
}
?>

class Strings
{
    public function count_of_each_letter($string){
        $string_chars = array();
        $length_ = mb_strlen($string,'UTF-8');
        if($length_== 0){return null;}
        else{
            for ($i=0; $i < $length_; $i++) { 
                $each_letter = mb_substr($string,0,1,'UTF-8');
                $string_chars[$each_letter] = mb_substr_count($string, $each_letter);
                $string = str_replace($each_letter,"", $string);
                $length_ = mb_strlen($string,'UTF-8');          
            }
            $string = '';
            foreach ($string_chars as $key => $value) { 
                $string .= $key.'-'.$value.'<br>';
            }
            return $string;
        }
    }
}
$new_counter  = new Strings();
echo $new_counter::count_of_each_letter('ختواجرایآهنگبهصورتتکنفرهنمود.اوازسال۱۹۷۲تا۱۹۷۵،۴آلبوماستودیوییتک‌نفرهمنتشرکردوحتینامزدیکجایزهاسکارهمشد.درهمینسال‌هاگروهاقدامبهبرگزاریتورکنسرتدراروپاونیزیکتورجهانیکردند.جکسونفایودرسال۱۹۷۵ازشرکتنشرموسیقیموتاونرکوردزبهسی‌بی‌اسرکوردزنقلمکانکردند.گروههمچنانبهاجراهایبین‌المللیخودادامهمی‌دادواز۱۹۷۶تا۱۹۸۴(از۱۵تا۲۴سالگیمایکل)ششآلبوماستودیوییدیگرمنتشرکرد.درهمینمدت،مایکلترانه‌سرایاصلیگروهجکسونزبود.Cantional,oderGesangbuchAugsburgischerKonfessionin1627.ohannSebastianBachcomposedafour-partsetting,BWV285,whichiswithouttext.twaspublishedasNo.196inthecollectionofchoralesbyJohannPhilippKirnbergerundCarlPhilippEmanufread');

You don't have to convert that into an array() you can use substr_count() to achieve the same.

substr_count — Count the number of substring occurrences

<?php
$str = "cdcdcdcdeeeef";
echo substr_count($str, 'c'); 
?>

PHP Manual

substr_count() returns the number of times the needle substring occurs in the haystack string. Please note that needle is case sensitive.

EDIT:

Sorry for the misconception, you can use count_chars to have a counted value of each character in a string. An example:

<?php
$str = "cdcdcdcdeeeef";

foreach (count_chars($str, 1) as $strr => $value) {
   echo chr($strr) . " occurred a number of $value times in the string." . "<br>";
}
?>

PHP Manual: count_chars

count_chars — Return information about characters used in a string