array_change_key_case()

PHP array_change_key_case() Function

array_change_key_case() returns a copy of an array with all of its string keys converted to either lowercase or uppercase. It is most useful for normalizing keys that come from an unpredictable source — HTTP headers, CSV column titles, database rows, or user input — so the rest of your code can look them up by a single, known casing.

This page covers the syntax, parameters, return value, the gotchas around numeric keys and key collisions, and a few practical recipes.

Syntax

array_change_key_case(array $array, int $case = CASE_LOWER): array

The function returns a new array with the keys re-cased; the original array is left untouched. Only string keys are affected — integer keys are returned unchanged (see Numeric keys are ignored below).

Basic example: uppercasing keys

Output:

Array
(
    [FIRST_NAME] => John
    [LAST_NAME] => Doe
)

The keys are now uppercase while the values (John, Doe) are untouched.

Default behavior: lowercasing

When you omit the $case argument it defaults to CASE_LOWER, so keys become lowercase. This is the most common use — normalizing mixed-case input before you read it:

<?php

$headers = array("Content-Type" => "text/html", "X-Powered-By" => "PHP");

// No second argument → CASE_LOWER
$normalized = array_change_key_case($headers);

echo $normalized["content-type"]; // text/html

Output:

text/html

Now you can always read $normalized["content-type"] regardless of how the header was originally cased.

Numeric keys are ignored

array_change_key_case() only touches string keys. Integer keys pass through unchanged, so an indexed array comes back exactly as it went in:

<?php

$mixed = array("Name" => "Ada", 0 => "zero", "Age" => 36);
print_r(array_change_key_case($mixed, CASE_UPPER));

Output:

Array
(
    [NAME] => Ada
    [0] => zero
    [AGE] => 36
)

Watch out for key collisions

Because PHP array keys must be unique, re-casing can make two distinct keys collide. When that happens, the later value wins and the earlier entry is silently dropped:

<?php

$array = array("Name" => "Ada", "name" => "Grace");
print_r(array_change_key_case($array, CASE_LOWER));

Output:

Array
(
    [name] => Grace
)

Both "Name" and "name" become "name", so only the last assignment (Grace) survives. If preserving every entry matters, check for case-insensitive duplicates before calling this function.

Only the top level is changed

This function is not recursive — nested array keys are left as-is. To change keys deeper in the structure you must walk the array yourself:

<?php

function changeKeyCaseRecursive(array $array, int $case = CASE_LOWER): array
{
    $result = array_change_key_case($array, $case);
    foreach ($result as $key => $value) {
        if (is_array($value)) {
            $result[$key] = changeKeyCaseRecursive($value, $case);
        }
    }
    return $result;
}

$data = array("User" => array("First" => "Ada"));
print_r(changeKeyCaseRecursive($data, CASE_UPPER));

Output:

Array
(
    [USER] => Array
        (
            [FIRST] => Ada
        )

)

Common use cases

• Normalize HTTP headers or query parameters so lookups don't depend on the sender's casing.
• Clean up CSV or spreadsheet column headers before mapping rows to your model.
• Make config or database keys consistent across code that was written by different people.

Related functions

• strtolower() and strtoupper() — change the case of a string value (this function uses them on keys).
• array_keys() and array_values() — extract an array's keys or values.
• array_flip() — swap keys and values.
• array_map() — transform array values with a callback.
• See PHP Arrays for a broader overview.

Practice