Unless it is an object, count casts its argument to an array, and

  count((array) "example")
= count(array("example"))
= 1

Use mb_strlen:

$count = mb_strlen( $string );

You could also use strlen, but you asked for the best way ;)

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(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

It is possible with substr_count().

I think you are not passing a value to it properly. Try it like this:

$string = '120201M, 121212M-1, 21121212M, 232323M-2, 32323K, 323232K-1';

echo substr_count($string, 'K-1'); //echo's 1

echo substr_count($string, 'K'); // echo's 2