Array walk function in PHP - Step-by-Step Execution

Concept Flow - Array walk function

Start with array

↓

Pick first element

↓

Apply callback function

↓

Move to next element

↓

More elements?

Yes→Repeat callback

No↓

End

The array walk function goes through each element of an array and applies a callback function to it one by one.

Execution Sample

PHP

<?php
$arr = [1, 2, 3];
array_walk($arr, function(&$v) { $v *= 2; });
print_r($arr);
?>

This code doubles each element in the array by walking through it and applying a function.

Execution Table

Step	Current Element (value)	Callback Action	Array State After Action
1	1	Multiply by 2 → 2	[2, 2, 3]
2	2	Multiply by 2 → 4	[2, 4, 3]
3	3	Multiply by 2 → 6	[2, 4, 6]
4	No more elements	Stop	[2, 4, 6]

💡 All elements processed, array walk ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
arr	[1, 2, 3]	[2, 2, 3]	[2, 4, 3]	[2, 4, 6]	[2, 4, 6]
v (current element)	1	2	4	6	6

Key Moments - 2 Insights

Why does the array change even though the callback uses a parameter?

Because the callback parameter is passed by reference (&$v), changes inside the callback affect the original array elements, as shown in steps 1-3 of the execution_table.

What happens if the callback does not modify the element?

The array stays the same because array_walk applies the callback but does not change elements unless the callback modifies them, as seen in the exit step where no changes occur.

Visual Quiz - 3 Questions

Look at the execution_table, what is the array state after step 2?

A [2, 2, 3]

B [2, 4, 3]

C [1, 2, 3]

D [2, 4, 6]

💡 Hint

Check the 'Array State After Action' column for step 2 in the execution_table.

At which step does the array element 3 become 6?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Current Element' and 'Callback Action' columns in the execution_table.

If the callback function did not use & (reference), what would happen to the array?

A Array elements would remain unchanged

B Array elements would still be doubled

C Array would be emptied

D Callback would cause an error

💡 Hint

Refer to variable_tracker and key_moments about passing by reference.

Concept Snapshot

array_walk(array, callback) applies callback to each element.
Callback can modify elements if parameter is passed by reference (&).
Array changes happen inside callback if references used.
No return value; original array is modified directly.
Useful for simple element-wise operations.

Full Transcript

The array_walk function in PHP takes an array and a callback function. It goes through each element of the array one by one. For each element, it runs the callback function. If the callback changes the element by reference, the original array changes. In the example, each number in the array is doubled. The execution table shows each step: starting with 1, doubling it to 2, then 2 to 4, and 3 to 6. After all elements are processed, the function ends. This helps understand how array_walk modifies arrays by applying a function to each element.