PHP array_fill_keys: Create Arrays with Specific Keys

The PHP array_fill_keys function helps you to build an array where each key has the same value.

What is the array_fill_keys Function in PHP?

The array_fill_keys function creates an array with keys that you set in advance. Each key in the array holds the same value that you choose.

Here is the syntax:

array_fill_keys($keys, $value)

• $keys: an array of keys that you want to use in the new array.
• $value: the value that each key will hold.

The function returns a new array. If a key repeats, the last one replaces the earlier one.

You use this function when you have a set of keys and you want them to share the same value. This works for quick setup of default values or to prepare arrays for data that will come later.

Here is a quick example:

$keys = ['apple', 'banana', 'orange'];
$result = array_fill_keys($keys, 0);
print_r($result);

The output is:

Array
(
    [apple] => 0
    [banana] => 0
    [orange] => 0
)

Here, the function makes an array where each fruit name holds the value 0. This is useful if you plan to count each fruit later.

Use the array_fill_keys with Arrays in PHP

Create an array with predefined keys and a single value:

$keys = [1, 2, 3, 4];
$array = array_fill_keys($keys, 'empty');
print_r($array);

This code creates a numeric-keyed array. Each number points to the word 'empty'.

Here is the output:

Array
(
    [1] => empty
    [2] => empty
    [3] => empty
    [4] => empty
)

Create an array with string keys and a set value:

$keys = ['north', 'south', 'east', 'west'];
$array = array_fill_keys($keys, 'unknown');
print_r($array);

This example builds a map direction array. Each direction starts with the same placeholder value 'unknown':

Array
(
    [north] => unknown
    [south] => unknown
    [east] => unknown
    [west] => unknown
)

Create a multi-dimensional array with php array_fill_keys:

$keys = ['user1', 'user2'];
$array = array_fill_keys($keys, ['status' => 'offline', 'score' => 0]);
print_r($array);

This creates an array where each user has a sub-array. The sub-array has a status and a score with initial values.

Here is the output:

Array
(
    [user1] => Array
        (
            [status] => offline
            [score] => 0
        )

    [user2] => Array
        (
            [status] => offline
            [score] => 0
        )

)

So, how to handle an empty keys array in php array_fill_keys?

If the $keys array is empty, the function returns an empty array.

$keys = [];
$array = array_fill_keys($keys, 'test');
var_dump($array);

The result is an empty array:

array(0) {
}

Use array_fill_keys with dynamic key lists from another array:

$source = ['id' => 1, 'name' => 'John', 'email' => 'john@example.com'];
$keys = array_keys($source);
$array = array_fill_keys($keys, null);
print_r($array);

Here, the keys come from another array. The new array holds the same keys but each value is null. The output:

Array
(
    [id] => 
    [name] => 
    [email] => 
)

The Difference Between array_fill and array_fill_keys

The array_fill creates an array with numeric keys that start from the index you set. While the array_fill_keys creates an array with specific keys from an array you provide.

Here is a table that shows you key differences:

Here is the use case:

• Use array_fill when you need numeric keys in sequence.
• Use array_fill_keys when you need custom keys like names or IDs.

Wrapping Up

In this article, you learned the use of array_fill_keys and how it differs from array_fill. You saw how to set values for many keys in one step. Here is a quick recap:

• array_fill_keys builds arrays from a given set of keys.
• You can use it for default values, templates, or structured data.
• The function works with both numeric and string keys.
• Empty key arrays result in empty arrays.

FAQs

$keys = array("a", "b", "c"); $result = array_fill_keys($keys, 0); print_r($result); 

$keys = array("x", "y"); $result1 = array_fill(0, 2, "test"); $result2 = array_fill_keys($keys, "test"); print_r($result1); print_r($result2); 

$keys = array(1, 2, 3); $result = array_fill_keys($keys, "value"); print_r($result); 

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