Convert a numeric string with no delimiters into an array of potentially multi-digit numbers

How can I split a string with no delimiters, for example $a = '89111213';, into an array of integers?

Desired result:

[8, 9, 11, 12, 13]

I generate that input number in a loop with concatenation like $a .= $somevariable;.

Update:

As @therefromhere suggests in his comment, instead of concatenating the numbers into one string,put them in an array directly:

$a = array();

for(...) {
    $a[] = $somevariable;

}

---

Here is something that works in a way.

Important:
Assumption: Numbers are concatenated in increasing order and start with a number < 10 :)

$str = "89111213";
$numbers = array();
$last = 0;
$n = '';
for($i = 0, $l = strlen($str);$i<$l;$i++) {
    $n .= $str[$i];
    if($n > $last || ($i == $l - 1)) {
        $numbers[] = (int) $n;
        $last = $n;
        $n = '';
    }
}

print_r($numbers);

prints

Array
(
    [0] => 8
    [1] => 9
    [2] => 11
    [3] => 12
    [4] => 13
)

Although this might work to some extend, it will fail in a lot of cases. Therefore:

How do you obtain 89111213 ? If you are generating it from the numbers 8,9,11, etc. you should generate something like 8,9,11,12,13.