replace every second comma of string using php

Question

I have a string of that displays like this:

1235, 3, 1343, 5, 1234, 1

I need to replace every second comma with a semicolon

i.e.

Answer 1

try this:

$s = '1235, 3, 1343, 5, 1234, 1';
$is_second = false;
for ($i = 0; $i < strlen($s); $i++) {
    if ($is_second && $s[$i] == ',') {
        $s[$i] = ';';
    } elseif ($s[$i] == ',') {
        $is_second = true;
    }
}
echo $s;