- array_count_values: documentation ( source)
- array_multisort: documentation ( source)
- print_r: documentation ( source)
- str_replace: documentation ( source)
- array_keys: documentation ( source)
- str_split: documentation ( source)
<?php
$str = "Hello World";
$array = str_split(str_replace(' ', '', $str), 1);
$array = array_count_values($array);
array_multisort(array_keys($array), SORT_NATURAL, $array);
print_r($array);