PHP array_fill_keys Function

last modified March 13, 2025

The PHP array_fill_keys function creates an array filled with specified keys and a single value. It's useful for initializing arrays.

Basic Definition

The array_fill_keys function fills an array with values, specifying keys. It returns a new array with the given keys and value.

Syntax: array_fill_keys(array $keys, mixed $value): array. The keys array provides the keys, and all elements get the same value.

Basic array_fill_keys Example

This shows how to create a simple array with string keys and a default value.

basic_array_fill_keys.php

<?php

$keys = ['apple', 'banana', 'orange'];
$fruitArray = array_fill_keys($keys, 'in_stock');

print_r($fruitArray);

This creates an array where each fruit is a key with 'in_stock' as value. The output shows all elements share the same value but have different keys.

Numeric Keys Example

Demonstrates using numeric keys to create an array with array_fill_keys.

numeric_keys.php

<?php

$numbers = [1, 2, 3, 4];
$numberArray = array_fill_keys($numbers, 'prime');

print_r($numberArray);

This creates an array with numeric keys 1 through 4, each having 'prime' as its value. The function works equally well with numeric and string keys.

Mixed Key Types

Shows how array_fill_keys handles different key types in the same array.

mixed_keys.php

<?php

$mixedKeys = ['name', 42, 3.14, true];
$mixedArray = array_fill_keys($mixedKeys, 'value');

print_r($mixedArray);

This example uses string, integer, float, and boolean keys. Note how PHP converts the boolean true to integer 1 when used as an array key.

Using Variables as Value

Demonstrates using a variable as the fill value with array_fill_keys.

variable_value.php

<?php

$status = 'pending';
$userIds = [101, 102, 103, 104];
$userStatuses = array_fill_keys($userIds, $status);

print_r($userStatuses);

Here we use a variable as the fill value. All user IDs get the current status value. Changing $status later won't affect the created array.

Complex Value Example

Shows how to use an array as the fill value with array_fill_keys.

complex_value.php

<?php

$categories = ['books', 'movies', 'music'];
$defaultData = [
    'count' => 0,
    'last_updated' => null
];
$inventory = array_fill_keys($categories, $defaultData);

print_r($inventory);

This creates a multi-dimensional array where each category has the same default data structure. Note that all elements reference the same array.

Best Practices

Key Uniqueness: Duplicate keys will be overwritten.
Reference Values: Objects/arrays are referenced, not copied.
Performance: Efficient for initializing large arrays.
Readability: Clearer than manual array initialization.

Source

PHP array_fill_keys Documentation

This tutorial covered the PHP array_fill_keys function with practical examples showing its usage for array initialization scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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