You can use lambda function as a second parameter:
<?php
array_walk($myArray, function(&$value, $key){
// if you want to change array values then "&" before the $value is mandatory.
});
?>
Example (multiply positive values by two):
<?php
$myArray = array(1, 2, 3, 4, 5);
array_walk($myArray, function(&$value, $index){
if ($value > 0) $value *= 2;
});
?>