// the test array 
$array = array(
  'php', 'arrays', 'are', 'cool'  
);
// some variable for testing:
$some_var = 'NEW';
// the function that get's called for each entry
function format_array_values(&$item, $key, $some_var) {
  $item = $some_var . ": $item (KEY: $key)<br/>";
}
// "walk" trough each array item and call the function:
// "format_array_values"
array_walk($array, 'format_array_values', $some_var);
// print the result:
print_r($array);
/*
The output will be:
Array
(
  [0] => NEW: php (KEY: 0)<br/>
  [1] => NEW: arrays (KEY: 1)<br/>
  [2] => NEW: are (KEY: 2)<br/>
  [3] => NEW: cool (KEY: 3)<br/>
)
*/