Inténtalo tú mismo - php array

PHP Source:

<!DOCTYPE html>
<html>
<body>

<?php
function myfunction(&$value,$key)
{
$value="yellow";
}
$a=array("a"=>"red","b"=>"green","c"=>"blue");
array_walk($a,"myfunction");
print_r($a);
?>

</body>
</html>