The Power of PHP: Understanding the array_fill_keys Function

array_fill_keys() is a built-in PHP function that builds a new associative array from a list of keys and a single value that every entry shares. It is the fastest way to pre-seed an array when you already know the keys you need but want them all to start from the same default — for example zeroing a set of counters, marking a list of items as false, or creating placeholder slots to fill in later.

This page covers what array_fill_keys() does, its exact behavior with duplicate and numeric keys, how it differs from array_fill() and array_combine(), and when to reach for it.

Syntax

array_fill_keys(array $keys, mixed $value): array

Return value: a new array whose keys come from $keys and whose values are all $value. Available since PHP 5.2.0.

Keys follow PHP's normal array-key rules: only integers and strings are valid. A string that looks like a decimal integer (such as "5") is cast to the integer 5, and true/false/null keys become 1/0/"".

Basic example

Pass the keys as the first argument and the shared fill value as the second:

The output is an associative array using your keys, each mapped to the same value:

Array
(
    [a] => value
    [b] => value
    [c] => value
)

This is handy whenever you have a set of keys but the values either don't matter yet or should all start the same — a common case is initializing counters to 0:

<?php

$letters = array("a", "b", "c");
$counts = array_fill_keys($letters, 0);
$counts["a"]++;
$counts["a"]++;
$counts["c"]++;
print_r($counts);

?>

Array
(
    [a] => 2
    [b] => 0
    [c] => 1
)

Using null, arrays, or objects as the value

The fill value can be any type. Use null to create placeholder slots you'll populate later:

<?php

$keys = array("id", "status");
$filled = array_fill_keys($keys, null);
print_r($filled);

?>

Array
(
    [id] =>
    [status] =>
)

When the value is an array or object it is copied by value into each key, so modifying one entry never affects the others:

<?php

$buckets = array_fill_keys(array("a", "b"), array());
$buckets["a"][] = 1;
print_r($buckets);

?>

Array
(
    [a] => Array
        (
            [0] => 1
        )

    [b] => Array
        (
        )

)

Duplicate and numeric keys

Because the result is a real PHP array, the keys obey normal array-key rules. Duplicate keys collapse into a single entry, and decimal-string keys become integers:

<?php

print_r(array_fill_keys(array("5", 5, "name"), null));

?>

Array
(
    [5] =>
    [name] =>
)

Here "5" and 5 are both stored as the integer key 5, so only one 5 entry remains. Keep this in mind if your key list might contain duplicates — the array will be shorter than the input.

array_fill_keys() vs. related functions

• array_fill() fills a range of sequential integer keys starting at a given offset, rather than from an explicit key list. Use it when you want 0, 1, 2, … style indexes.
• array_combine() pairs a keys array with a values array of equal length, giving each key a different value. Use array_fill_keys() when every value is the same.
• array_keys() does the inverse: it extracts the keys out of an existing array.

A common pattern is to seed defaults with array_fill_keys() and then merge real data over the top:

<?php

$defaults = array_fill_keys(array("name", "email", "active"), null);
$input = array("name" => "Ann", "active" => true);
print_r(array_merge($defaults, $input));

?>

Array
(
    [name] => Ann
    [email] =>
    [active] => 1
)

This guarantees every expected key is present even when the incoming data is partial.

Summary

array_fill_keys() turns a list of keys plus one shared value into a ready-made associative array. Reach for it to initialize counters, flags, or placeholder records in a single line. Remember that the fill value is copied into every key, and that duplicate or decimal-string keys collapse following PHP's standard array-key rules. For different-per-key values use array_combine(); for sequential integer indexes use array_fill(). See the PHP arrays guide for the bigger picture.

Practice