array_walk() in PHP

PHP array_walk() Function:
array_walk() is not affected by the internal array pointer of array.
array_walk() will walk through the entire array regardless of pointer position.
array_walk() Apply a user supplied function to every member of an array

Syntax:
bool array_walk ( array &$array , callable $callback [, mixed $userdata = NULL ] )

Parameters:
    array:The input array.
    callback:Typically, callback takes on two parameters. The array parameter's value being the first, and the key/index second.
   
    Note:If callback needs to be working with the actual values of the array, specify the first parameter of callback as 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.
    Return Values:Returns TRUE on success or FALSE on failure.
    Errors/Exceptions: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.

<?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');
?>

Output:

Before ...:
d. lemon
a. orange
b. banana
c. apple
... and after:
d. fruit: lemon
a. fruit: orange
b. fruit: banana
c. fruit: apple

 Another example to array_walk() function:

<?php
function myfunc1($val,$key)
{
echo "The key $key has the value $val";
}
$daysarr1 = array( "a"=>"Sunday","b"=>"Monday","c"=>"Tuesday" );
array_walk($daysarr1,"myfunc1");
echo "<br />"."using parameter";
function myfunc2($val,$key,$par)
{
echo "$key $par $val";
}
$daysarr1 = array( "a"=>"Sunday","b"=>"Monday","c"=>"Tuesday" );
array_walk($daysarr1,"myfunc2","has the value");
echo "<br />"."changing an array element’s value";
function myfunc3(&$val,$key)
{
$val = "Wednesday";
}
$daysarr1 = array( "a"=>"Sunday","b"=>"Monday","c"=>"Tuesday" );
array_walk($daysarr1,"myfunc3");
print_r($daysarr1);
?>

The output of the code above will be:

The key a has the value Sunday 
The key b has the value Monday
The key c has the value Tuesday

 using parameter 
a has the value Sunday
 b has the value Monday 
c has the value Tuesday
changing an array element’s value 
Array (
        [a] => Wednesday
        [b] => Wednesday 
        [c] => Wednesday
       )