- floor: documentation ( source)
- explode: documentation ( source)
<?php
$a = "3.11";
$b = "2.9";
$c = "3.10";
$d = "1.11";
$arr1 = explode(".", $a);
$arr2 = explode(".", $b);
$arr3 = explode(".", $c);
$arr4 = explode(".", $d);
$y = $arr1[0] + $arr2[0] + $arr3[0] + $arr4[0]; // calculate years
If($arr1[1] + $arr2[1] + $arr3[1] + $arr4[1] >=12) $y += floor(($arr1[1] + $arr2[1] + $arr3[1] + $arr4[1])/12); // add one if months is more than 11
$m = ($arr1[1] + $arr2[1] + $arr3[1] + $arr4[1])%12; // modulo calculate months
Echo $y . " " . $m;