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

Concept Flow - Array filter function

Start with array

↓

Pick first element

↓

Apply filter function

↓

Keep element

↓

Move to next element

↓

More elements?

No→Return filtered array

↩Back to Pick first element

The filter function checks each element with a test. If the test passes, it keeps the element; if not, it skips it. This repeats until all elements are checked.

Execution Sample

PHP

<?php
$numbers = [1, 2, 3, 4, 5];
$even = array_filter($numbers, fn($n) => $n % 2 === 0);
print_r($even);
?>

This code filters the array to keep only even numbers.

Execution Table

Step	Current Element	Condition ($n % 2 === 0)	Keep Element?	Filtered Array So Far
1	1	1 % 2 === 0 → false	No	[]
2	2	2 % 2 === 0 → true	Yes	[2]
3	3	3 % 2 === 0 → false	No	[2]
4	4	4 % 2 === 0 → true	Yes	[2, 4]
5	5	5 % 2 === 0 → false	No	[2, 4]

💡 All elements checked, filter complete.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
$numbers	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]
$even	[]	[]	[2]	[2]	[2,4]	[2,4]	[2,4]

Key Moments - 3 Insights

Why does the filtered array only contain some elements, not all?

Because the filter function tests each element. Only elements passing the condition (true) are kept, as shown in execution_table rows 2 and 4.

Does the original array change after filtering?

No, the original array ($numbers) stays the same throughout, as shown in variable_tracker where $numbers never changes.

What happens if the condition is false for all elements?

The filtered array will be empty because no elements pass the test, similar to execution_table rows where 'Keep Element?' is 'No'.

Visual Quiz - 3 Questions

Look at the execution table, what is the filtered array after step 4?

A [2, 3, 4]

B [2, 4]

C [1, 3, 5]

D [1, 2, 3, 4]

💡 Hint

Check the 'Filtered Array So Far' column at step 4 in the execution_table.

At which step does the condition first become true?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Condition' column in execution_table to find the first 'true' result.

If the filter condition was changed to keep odd numbers, what would be the filtered array after step 5?

A [1, 3, 5]

B [2, 4]

C [1, 2, 3, 4, 5]

D []

💡 Hint

Think about which numbers are odd in the original array $numbers from variable_tracker.

Concept Snapshot

array_filter(array, callback) → returns new array
Checks each element with callback function
Keeps elements where callback returns true
Original array stays unchanged
Useful to select elements by condition

Full Transcript

The array_filter function in PHP takes an array and a callback function. It checks each element of the array with the callback. If the callback returns true, the element is kept in the new filtered array. If false, the element is skipped. This process repeats for all elements. The original array does not change. For example, filtering even numbers keeps only numbers divisible by 2. The filtered array builds up step by step as elements pass the test.