This function works just like array_walk() with the added bonus of calling array_walk() automatically on any element of your array which is an array.

$arr = array( 'one'   => array( 'A' => 'Alligator',
                                'a' => 'apple' ),
              'two'   => array( 'B' => 'Bull',
                                'b' => 'banana' ),
              'three' => array( 'C' => 'Crab',
                                'c' => 'carrot' ),
              'four'  => array( 'D' => 'Dingo',
                                'd' => 'durian' ),
              'na'    => 'Not an Array' );
 
function isFor( $word, $letter ) {
  echo "$letter is for $word\n";
}
array_walk_recursive( $arr, 'isFor' );
/*
A is for Alligator
a is for apple
B is for Bull
b is for banana
C is for Crab
c is for carrot
D is for Dingo
d is for durian
na is for Not an Array
*/

Some versions of PHP had a bug which caused the array to be altered. Some of the keys would be transformed into references which could cause your data to be changed without you realizing it.

See the manual entry for array_walk_recursive()