PHP array_key_exists Function

last modified March 13, 2025

The PHP array_key_exists function checks if a specified key exists in an array. It's essential for safe array access and validation.

Basic Definition

The array_key_exists function verifies if a key is present in an array. It returns true if the key exists, false otherwise.

Syntax: array_key_exists(string|int $key, array $array): bool. The function works with both string and integer keys in any array type.

Basic array_key_exists Example

This demonstrates checking for a key in a simple associative array.

basic_array_key_exists.php

<?php

$user = [
    'name' => 'John',
    'age' => 30,
    'email' => 'john@example.com'
];

if (array_key_exists('email', $user)) {
    echo "Email exists: {$user['email']}";
} else {
    echo "Email not found";
}

This checks if the 'email' key exists in the $user array. Since it does, the code outputs the email address. Always validate keys before access.

Checking Numeric Keys

The function works equally well with numeric array indices.

numeric_keys.php

<?php

$colors = ['red', 'green', 'blue'];

if (array_key_exists(1, $colors)) {
    echo "Second color is: {$colors[1]}"; 
}

if (!array_key_exists(3, $colors)) {
    echo "Index 3 doesn't exist";
}

This verifies numeric indices in a sequential array. Note that PHP arrays are zero-indexed, so index 1 refers to the second element.

Difference from isset()

array_key_exists differs from isset in handling null values.

isset_comparison.php

<?php

$data = [
    'name' => 'Alice',
    'age' => null
];

var_dump(array_key_exists('age', $data)); // bool(true)
var_dump(isset($data['age']));           // bool(false)

array_key_exists returns true for null values, while isset returns false. Choose based on whether you need to detect null as valid.

Multi-dimensional Arrays

The function can check keys in nested array structures.

multi_dimensional.php

<?php

$inventory = [
    'fruits' => [
        'apple' => 10,
        'banana' => 15
    ],
    'vegetables' => [
        'carrot' => 20
    ]
];

if (array_key_exists('fruits', $inventory) && 
    array_key_exists('banana', $inventory['fruits'])) {
    echo "Banana count: {$inventory['fruits']['banana']}";
}

This checks keys at multiple levels. First verify the parent key exists, then check the nested key to avoid undefined index errors.

Performance Considerations

The function has constant time complexity (O(1)) for hash table lookups.

performance.php

<?php

$largeArray = array_fill(0, 1000000, 'value');

$start = microtime(true);
array_key_exists(999999, $largeArray);
$time = microtime(true) - $start;

echo "Key check took: " . number_format($time * 1000, 3) . " ms";

Even with one million elements, key lookup remains fast. The function uses PHP's internal hash table implementation for efficient searches.

Best Practices

Pre-validation: Use before accessing array elements.
Type safety: Works with both string and integer keys.
Null handling: Prefer over isset() when null is valid.
Readability: Makes code intentions clear.

Source

PHP array_key_exists Documentation

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

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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