PHP array_keys: How to Extract Keys in Arrays with Examples

This guide shows how PHP array_keys finds all keys in an array and how you can filter them by a value.

What is PHP array_keys?

The PHP array_keys function finds all keys in an array. It takes an array and returns its keys in a new array. You can also pass an optional value and strict mode to filter the keys.

Here is the syntax:

array_keys($array, $value = null, $strict = false)

• The first parameter is the array that holds data.
• The second parameter is an optional value to match.
• The third parameter is strict mode. It matches keys only with the same type and value.

Here is an example:

$data = ["a" => 10, "b" => 20, "c" => 10];
$keys = array_keys($data);
print_r($keys);

This code takes an array with three elements and returns all keys. It prints ["a","b","c"].

You can pass a value as the second parameter. PHP will return only keys that hold that value.

$data = ["a" => 10, "b" => 20, "c" => 10];
$keys = array_keys($data, 10);
print_r($keys);

This code returns only keys with value 10. It prints ["a","c"].

Examples of the array_keys Function in PHP

Basic Use:

$items = ["pen" => 5, "book" => 2, "bag" => 1];
$keys = array_keys($items);
print_r($keys);

This code takes an array with three items. It returns all keys from that array. The output is ["pen","book","bag"]. This shows the base use of array_keys.

Filter Keys by Value:

$grades = ["Ali" => 90, "Sara" => 80, "Omar" => 90];
$keys = array_keys($grades, 90);
print_r($keys);

This code takes a list of names and scores. It returns only the names with score 90. The output is ["Ali","Omar"]. This shows how to filter keys by value.

Use Strict Mode:

$data = ["x" => "5", "y" => 5, "z" => 5];
$keys = array_keys($data, 5, true);
print_r($keys);

This code holds numbers and strings. It returns only keys with value and type equal to 5. The output is ["y","z"]. This shows strict comparison with array_keys.

Nested Array Keys:

$data = ["user1" => ["age" => 20], "user2" => ["age" => 25]];
$keys = array_keys($data);
print_r($keys);

This code has a nested array of users. It returns the main keys from that array. The output is ["user1","user2"]. This shows that array_keys does not go deep but only reads the first level.

Wrapping Up

You learned what PHP array_keys does and how it collects keys from arrays. You also learned how to filter keys by value, how to use strict mode, and how it differs from array_values.

Here is a quick recap:

• array_keys collects keys from arrays.
• You can filter keys by value or by strict match.
• array_values collects values from arrays.

FAQs

<?php
$fruits = array("a" => "Apple", "b" => "Banana", "c" => "Apple");
print_r(array_keys($fruits));
print_r(array_keys($fruits, "Apple"));
?>

array_keys(array $array, mixed $search_value = null, bool $strict = false): array

<?php
$students = array("id1" => "John", "id2" => "Alice", "id3" => "John");
$result = array_keys($students, "John");
print_r($result);
?>

<?php
$data = array("x" => 100, "y" => 200);
print_r(array_keys($data));   // Outputs: Array ( [0] => x [1] => y )
print_r(array_values($data)); // Outputs: Array ( [0] => 100 [1] => 200 )
?>

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