The function strtotime() returns the time in seconds since January 1 1970 00:00:00 UTC. So adding the return value of this function might not do what you would expect.
Instead of using the date functions we can manipulate the string and perform some basic arithmetic operations:
<?php
$value1 = "12:44";
$value2 = "13:47";
$arr1 = explode(':', $value1);
$arr2 = explode(':', $value2);
$totalMinutes = (int)$arr1[0] * 60 + (int)$arr1[1] + (int)$arr2[0] * 60 + (int)$arr2[1];
$hours = (int) ($totalMinutes / 60);
$minutes = $totalMinutes % 60; // Modulus: remainder when dividing with 60
echo $hours . ':' . $minutes;
?>