Array filter function in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the array_filter() function do in PHP?

It filters elements of an array using a callback function, returning a new array with only the elements that pass the test.

beginner

How do you use array_filter() without a callback function?

It removes all elements that are considered false (like 0, '', null, false) and returns the rest.

beginner

What type of argument does the callback function in array_filter() receive?

The callback function receives each element of the array as its argument to test if it should be included.

intermediate

Does array_filter() preserve the original array keys?

Yes, it preserves the keys of the original array in the filtered result.

intermediate

How can you reindex the array returned by array_filter()?

Use the array_values() function to reset the keys to consecutive numbers starting from 0.

What does array_filter() return when no callback is provided?

A An empty array

B The original array unchanged

C An array with all elements except those that are falsey

D An array with only falsey elements

Without a callback, array_filter() removes falsey values like 0, '', false, and null.

Which of these is a valid callback for array_filter()?

A A function that returns true or false based on the element

B A function that returns the element itself

C A function that returns a string

D A function that returns an integer index

The callback must return true to keep the element or false to remove it.

Does array_filter() change the keys of the original array by default?

A Yes, it resets keys to 0-based indexes

B No, it preserves the original keys

C It removes keys completely

D It converts keys to strings

array_filter() keeps the original keys unless you reindex manually.

How do you remove all empty strings from an array using array_filter()?

A Use <code>array_filter($array, fn($v) =&gt; $v !== '')</code>

B Use <code>array_filter($array, fn($v) =&gt; is_null($v))</code>

C Use <code>array_filter($array, fn($v) =&gt; $v === '')</code>

D Use <code>array_filter($array)</code> without a callback

The callback keeps elements that are not empty strings.

What function can you use to reindex an array after filtering?

A array_keys()

B array_merge()

C array_slice()

D array_values()

array_values() resets keys to consecutive numbers starting at 0.

Explain how array_filter() works and give an example of filtering out falsey values.

Think about how you pick only certain items from a list based on a test.

Describe how to preserve or reset keys when using array_filter().

Consider what happens to the numbering of items after filtering.