The following code stops at 254, whereas the documentation says count_chars ought to generate an array of 256 element.

var_dump($b) shows an array with 255 elements instead of 256.

$a = "a quick brown fox jumps over the lazy dog";
$b = count_chars($a, 0);
$m = 0;
foreach ($b as $n) {
 echo "Code $m was found $n times<br>";
 $m++;
}