Substr(). Add characters for each n digits [closed]

Answer 1

Split string on every 18th position. Then add space on every value of the array on 10th position.

$k = "2013-01-0112:00:002013-01-0212:00:002013-01-0312:00:00";
$a = str_split($k, 18);
$a = array_map(function($dt) {
    return implode(' ', str_split($dt, 10));
}, $a);
print_r($a);

Demo.

---

If you wish to add character to N-th position:

$k = "2013-01-0112:00:002013-01-0212:00:002013-01-0312:00:00";
echo implode(',', str_split($k, 18));
//            ^                  ^
//        character         N-th position

Demo.