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

Preg_replace() solution

$str = '1235, 3, 1343, 5, 1234, 1';
$str = preg_replace('/(.+?),(.+?),/', '$1,$2;', $str);
echo $str;

Output:

1235, 3; 1343, 5; 1234, 1