array_walk()

(PHP 4, PHP 5, PHP 7)

使用用户自定义函数对数组中的每个元素做回调处理

说明

将用户自定义函数$funcname应用到$array数组中的每个单元。

array_walk()不会受到$array内部数组指针的影响。array_walk()会遍历整个数组而不管指针的位置。

参数

输入的数组。

典型情况下$callback接受两个参数。$array参数的值作为第一个，键名作为第二个。

如果$callback需要直接作用于数组中的值，则给$callback的第一个参数指定为引用。这样任何对这些单元的改变也将会改变原始数组本身。Note:

参数数量超过预期，传入内置函数(例如strtolower())，将抛出警告，所以不适合当做$funcname。

只有$array的值才可以被改变，用户不应在回调函数中改变该数组本身的结构。例如增加/删除单元，unset 单元等等。如果array_walk()作用的数组改变了，则此函数的的行为未经定义，且不可预期。

如果提供了可选参数$userdata，将被作为第三个参数传递给callback$funcname。

返回值

成功时返回TRUE，或者在失败时返回FALSE。

错误／异常

如果$callback函数需要的参数比给出的多，则每次array_walk()调用$callback时都会产生一个E_WARNING级的错误。

范例

Example #1array_walk()例子

以上例程会输出：

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

参见

• array_walk_recursive() 对数组中的每个成员递归地应用用户函数
• iterator_apply() 为迭代器中每个元素调用一个用户自定义函数
• list() 把数组中的值赋给一组变量
• each() 返回数组中当前的键／值对并将数组指针向前移动一步
• call_user_func_array() 调用回调函数，并把一个数组参数作为回调函数的参数
• array_map() 为数组的每个元素应用回调函数
• callback类型的信息
• foreach

It's worth nothing that array_walk can not be used to change keys in the array.
The function may be defined as (&$value, $key) but not (&$value, &$key).
Even though PHP does not complain/warn, it does not modify the key.

I noticed that :
PHP ignored arguments type when using array_walk() even if there was
 
declare(strict_types=1) . 
See this code as an example ...

The output is :
butter: 5
meat: 7
banana: 3
whilst the expecting output is :
Fatal error: Uncaught TypeError: Argument 1 passed to test_print() must be of the type integer
because "butter" => 5.3 is float
I asked someone about it and they said "this was caused by the fact that callbacks called from internal code will always use weak type". But I tried to do some tests and this behavior is not an issue when using call_user_func().

Calling an array Walk inside a class 
If the class is static:
array_walk($array, array('self', 'walkFunction'));
or
array_walk($array, array('className', 'walkFunction'));
Otherwise:
array_walk($array, array($this, 'walkFunction'));

If you want to unset elements from the callback function, maybe what you really need is array_filter.

Since array_walk cannot modify / change / reindex keys as already mentioned, i provide this small wrapping function which accomplishes passing array reference and index using closures , "use" keyword.
function indexArrayByElement($array, $element)
{
  $arrayReindexed = [];
  array_walk(
    $array,
    function ($item, $key) use (&$arrayReindexed, $element) {
      $arrayReindexed[$item[$element]] = $item;
    }
  );
  return $arrayReindexed;
}

// We can make that with this simple FOREACH loop : 
$fruits = array("d" => "lemon", "a" => "orange", "b" => "banana", "c" => "apple");
foreach($fruits as $cls => $vls)
{
 $fruits[$cls] = "fruit: ".$vls;
}
Results: 
Array
(
  [d] => fruit: lemon
  [a] => fruit: orange
  [b] => fruit: banana
  [c] => fruit: apple
)

It can be very useful to pass the third (optional) parameter by reference while modifying it permanently in callback function. This will cause passing modified parameter to next iteration of array_walk(). The exaple below enumerates items in the array:

This outputs:
Array
(
  [0] => 1 lemon
  [1] => 2 orange
  [2] => 3 banana
  [3] => 4 apple
)
$num is: 1
Notice at the last line of output that outside of array_walk() the $num parameter has initial value of 1. This is because array_walk() does not take the third parameter by reference.. so what if we pass the reference as the optional parameter.. 

There is a note about 3 years ago regarding using this for trimming. array_map() may be cleaner for this. I haven't checked the time/resource impact:
$result = array_map("trim", $array);

Don't forget about the array_map() function, it may be easier to use!
Here's how to lower-case all elements in an array: 

Unfortunately I spent a lot of time trying to permanently apply the effects of a function to an array using the array_walk function when instead array_map was what I wanted. Here is a very simple though effective example for those who may be getting overly frustrated with this function... 

array_walk does not work on SplFixedArray objects:

result is:
test_1
test_2

You can use lambda function as a second parameter:

Example (multiply positive values by two): 

Using lambdas you can create a handy zip function to zip together the keys and values of an array. I extended it to allow you to pass in the "glue" string as the optional userdata parameter. The following example is used to zip an array of email headers:

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