PHP array_walk Function | Iterating Array Elements

The PHP array_walk Function loops over every element of array and applies callback function to them, hence saving us the effort of writing extra loops. In this article, we will discuss the array_walk function. Also, we will discuss a few examples of using it.

Array_walk function iterates over every element of the array in the call-back function/user-defined function. It returns true or false as output and can also modify the array using call by reference. It only deals with single dimensional arrays, it does not work with multi-dimensional arrays.

Syntax

array_walk($array , $callbackFunction , $param);

Parameters

PHP array_walk Function expects two mandatory parameters and one optional parameter. Always supply at most three parameters and a minimum of two parameters to array_walk function otherwise it will throw an error. 

• $array: It is the array on which callback function is applied.
• $callbackFunction: It is the user-defined function which is applied to the array.
• $param: The extra parameter for the callback function, it is optional.

Usage and Examples

Now let us see some examples to get better understanding of PHP array_walk Function.

Note: PHP array_walk Function does not work on pre-defined functions like strtolower etc.

Example 1: Check Odd/Even Values

For instance, you want to check for even and odd values in the array and display them. Typically, you will be writing the below code:

<?php
$array = array(10, 45, 100, 57, 28, 60, 17);

foreach ($array as $key => $value) {
        if ($value % 2 == 0) {
            echo $value . " is an even number";
            echo "</br>";
        } else {
            echo $value . " is an odd number";
            echo "</br>";
            }
        }

OUTPUT:
 10 is an even number
 45 is an odd number
 100 is an even number
 57 is an odd number
 28 is an even number
 60 is an even number
 17 is an odd number

Now, we will be using PHP array_walk Function to see another way of implementation the above code.

function checkEvenOddNumber($value,$key)
{
  if($value % 2 == 0)
    {
      echo $value." is an even number";
      echo "</br>";
    }
  else
    {
      echo  $value." is an odd number";
      echo "</br>";
    }
}

$array = array(10,45,100,57,28,60,17);
array_walk($array,"checkEvenOddNumber");

OUTPUT:
 10 is an even number
 45 is an odd number
 100 is an even number
 57 is an odd number
 28 is an even number
 60 is an even number
 17 is an odd number

Here, no changes were made to the original array because all we did is just to check for even and odd values. Let’s see another example where PHP array_walk Function modifies the original array through call by reference.

Example 2: Multiplying Every Element of Array

We are taking a very simple example of multiplying every element of the array by the given number. We emphasize on the fact of the call by reference.

function multiplyByNum(&$value,$key,$num)
{
  return $value = $value*$num;
}

$array = array(1,2,3,4,5);
array_walk($array,"multiplyByNum",'3');

echo "<pre>";
print_r($array);
echo "</pre>";

OUTPUT:
 Array
 (
     [0] => 3
     [1] => 6
     [2] => 9
     [3] => 12
     [4] => 15
 )

Only $values can be changed by reference but not $keys! Even if you pass &$keys as parameter, they will remain unchanged!

Example 3: Using Method Of A Class

We can also call a class method in array_walk function in PHP. We simply create an object of the class and pass method of the instantiated object as an array. The array contains instantiated object at 0th index and method name which you wish to call at index 1.

Syntax

array_walk($array, array( $obj , ‘mehodName’) );

Let us quickly see it using below example. Here we call a method cubeOfNumbera of class testArrayWalk. We create an object of class $classObj and pass it along with ‘cubeOfNumbers’ as an array to PHP array_walk function.

 class testArrayWalk
  {
    
    public function cubeOfNumbers(&$value,$key)
        {
          $value = $value*$value*$value;
        }
  }


$classObj = new testArrayWalk();
print_r($classObj);
$array = array(1,2,3,4);

array_walk($array,array($classObj, 'cubeOfNumbers'));
echo "<pre>";
print_r($array);
echo "</pre>";

OUTPUT:
 Array
 (
     [0] => 1
     [1] => 8
     [2] => 27
     [3] => 64
 )

You can read more about Classes and follow our tutorial on Object Oriented Concepts in PHP.

Example 4: Using Lambda Function

PHP array_walk function also works well with lambda functions. Here we are calculating square of every number of the array by multiplying values with themselves. Function along with its logic is being passed as second parameter to array_walk function.

$array = array(1,2,3,4,5);
array_walk($array, function(&$value,$key)
       {
             return $value = $value*$value;   
       }
      );

echo "<pre>";
print_r($array);
echo "</pre>";

Array
 (
     [0] => 1
     [1] => 4
     [2] => 9
     [3] => 16
     [4] => 25
 )

Performance Analysis

I carried out a performance analysis test on array_walk and the foreach loop. I ran tests based on the examples given above in their respective order. Same source code has been used as used in examples above with their foreach counterparts and array_walk.

It is evident from the above graph that foreach runs faster than array_walk function. The slowdown in array_walk function is caused by an extra function call. So it is suggested to use the foreach loop instead of array_walk function. In fact, for some cases, you can also use thearray_map function.

Conclusion

In this article, we discussed the use of PHP array_walk function for looping over elements in an array. We also did a performance analysis test for conventional foreach vs array_walk.

You can also read more about array_walk function on the official PHP documentation. Please read more articles on Array Functions in PHP on Concatly. Kindly leave your valuable comments below to help me improve this resource.