(PHP 4, PHP 5, PHP 7)
array_walk — Apply a user supplied function to every member of an array
   Applies the user-defined callback function to each
   element of the array array.
  
   array_walk() is not affected by the internal array
   pointer of array.  array_walk()
   will walk through the entire array regardless of pointer position.
  
arrayThe input array.
callback
       Typically, callback takes on two parameters.
       The array parameter's value being the first, and
       the key/index second.
      
Note:
If
callbackneeds to be working with the actual values of the array, specify the first parameter ofcallbackas a reference. Then, any changes made to those elements will be made in the original array itself.
Note:
Many internal functions (for example strtolower()) will throw a warning if more than the expected number of argument are passed in and are not usable directly as a
callback.
       Only the values of the array may potentially be
       changed; its structure cannot be altered, i.e., the programmer cannot
       add, unset or reorder elements. If the callback does not respect this
       requirement, the behavior of this function is undefined, and      
       unpredictable.
      
userdata
       If the optional userdata parameter is supplied,
       it will be passed as the third parameter to the
       callback.
      
   Returns TRUE on success or FALSE on failure.
  
   If function callback requires more parameters than
   given to it, an error of level 
   E_WARNING will be generated each time array_walk()
   calls callback.
  
Example #1 array_walk() example
<?php
$fruits = array("d" => "lemon", "a" => "orange", "b" => "banana", "c" => "apple");
function test_alter(&$item1, $key, $prefix)
{
    $item1 = "$prefix: $item1";
}
function test_print($item2, $key)
{
    echo "$key. $item2<br />\n";
}
echo "Before ...:\n";
array_walk($fruits, 'test_print');
array_walk($fruits, 'test_alter', 'fruit');
echo "... and after:\n";
array_walk($fruits, 'test_print');
?>
The above example will output:
Before ...: d. lemon a. orange b. banana c. apple ... and after: d. fruit: lemon a. fruit: orange b. fruit: banana c. fruit: apple