<?php
function array_transpose($arr) {
   $map_args = array_merge(array(NULL), $arr);
   return call_user_func_array('array_map', $map_args);
}

$arr = array( 
  0 => array(0 => 0, 1 => 20, 2 => 5), 
  1 => array(0 => 5, 1 => 0,  2 => 15), 
  2 => array(0 => 5, 1 => 10, 2 => 0)
);

$maxes = array_map('max', array_transpose($arr));

print_r($maxes);
?>

Array
(
    [0] => 5
    [1] => 20
    [2] => 15
)