applying a call back function to all keys on an array.

 $array1 = array('blue'  => 1, 'red'  => 2, 'green'  => 3, 'purple' => 4);

$array2 = array('green' => 3, 'blue1' => 1, 'yellow' => 7, 'cyan'   => 8);

echo "<br>";

print_r(array_map('strtoupper',array_keys($array1)));  

// will print Array ( [0] => BLUE [1] => RED [2] => GREEN [3] => PURPLE )

echo "<br>";