PHP array_key_first Function

last modified March 13, 2025

The PHP array_key_first function retrieves the first key of an array. It's useful when you need to access the initial element without resetting the array pointer.

Basic Definition

The array_key_first function returns the first key of the given array. It works with both indexed and associative arrays.

Syntax: array_key_first(array $array): int|string|null. Returns null for empty arrays. Available since PHP 7.3.

Basic array_key_first Example

This demonstrates getting the first key from a simple associative array.

basic_array_key_first.php

<?php

$colors = [
    'red' => '#FF0000',
    'green' => '#00FF00',
    'blue' => '#0000FF'
];

$firstKey = array_key_first($colors);

echo "First color key: $firstKey";

The function returns 'red' as it's the first key in the array. The original array remains unchanged.

Indexed Array Example

array_key_first works with numeric indexes as well.

indexed_array.php

<?php

$fruits = ['apple', 'banana', 'cherry'];
$firstIndex = array_key_first($fruits);

echo "First fruit index: $firstIndex";

For indexed arrays, it returns 0 (the first numeric index). This matches PHP's standard array indexing behavior.

Empty Array Handling

The function returns null when called with an empty array.

empty_array.php

<?php

$emptyArray = [];
$result = array_key_first($emptyArray);

var_dump($result);

This behavior helps distinguish between arrays with a null key and truly empty arrays. Always check for null when working with unknown arrays.

Mixed Key Types

The function handles arrays with mixed key types (numeric and string).

mixed_keys.php

<?php

$mixed = [
    10 => 'ten',
    'color' => 'blue',
    20 => 'twenty'
];

$firstKey = array_key_first($mixed);

echo "First key: "; 
var_dump($firstKey);

Despite having string keys later, the function returns the first key (10). The type (int) is preserved in the return value.

Practical Usage Example

Here's how to use array_key_first in a real-world scenario.

practical_usage.php

<?php

$userPreferences = [
    'theme' => 'dark',
    'language' => 'en',
    'notifications' => true
];

$mainPreference = array_key_first($userPreferences);

echo "Main preference setting: $mainPreference";

This shows getting the primary preference key without affecting the array. Useful when you need the first setting but want to preserve the array.

Best Practices

Null Checks: Always verify the return value isn't null.
PHP Version: Ensure PHP 7.3+ for this function.
Readability: Prefer over reset()+key() for clarity.
Performance: Faster than array_keys()[0] for large arrays.

Source

PHP array_key_first Documentation

This tutorial covered the PHP array_key_first function with practical examples showing its usage for array key retrieval.

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.

List all PHP Array Functions.

Ebooks