The array_keys() function in PHP is used to retrieve all the keys from an array and return them in a new array. You can optionally filter the keys by specifying a particular value to return only the keys that correspond to that value. This function works for both associative arrays (with string keys) and indexed arrays (with numeric keys).

Syntax:

array_keys(array $array, mixed $value = null, bool $strict = false): array

Parameters:

1. $array: The input array from which to retrieve the keys.
2. $value (optional): If specified, array_keys() will return only the keys that have this value.
3. $strict (optional): If set to true, the function will use strict comparison (===) to compare values, meaning both the value and its type must match exactly. The default is false (loose comparison).

Return Value:

• The function returns an indexed array of keys from the input array. If $value is provided, only keys associated with that value will be returned.

Example 1: Retrieving All Keys

In its basic form, array_keys() returns all the keys from an array.

<?php
$array = [
    "name" => "Alice",
    "age" => 25,
    "email" => "alice@example.com"
];

$keys = array_keys($array);
print_r($keys);
?>

Output:

Array
(
    [0] => name
    [1] => age
    [2] => email
)

In this example, the function returns an array containing all the keys ("name", "age", and "email") from the input associative array.

Example 2: Filtering Keys by Value

You can specify a value, and array_keys() will return only the keys that are associated with that value.

<?php
$array = [10, 20, 10, 30, 10];

$keys = array_keys($array, 10);
print_r($keys);
?>

Output:

Array
(
    [0] => 0
    [1] => 2
    [2] => 4
)

Here, array_keys() returns only the keys where the value is 10. These keys are 0, 2, and 4.

Example 3: Strict Comparison (===)

By default, array_keys() performs loose comparison (==) when comparing values. However, you can enable strict comparison (===) by setting the third parameter to true.

<?php
$array = [
    "a" => 1,
    "b" => "1",
    "c" => 1.0
];

// Using loose comparison (default)
$keys = array_keys($array, 1);
print_r($keys); // All values loosely equal to 1 are matched

// Using strict comparison
$strictKeys = array_keys($array, 1, true);
print_r($strictKeys); // Only strictly equal (1 === 1) is matched
?>

Output (Loose Comparison):

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

Output (Strict Comparison):

Array
(
    [0] => a
)

In the loose comparison, array_keys() considers all values (1, "1", 1.0) to be equal to 1, so it returns all the keys. With strict comparison, only the exact match (1 === 1) is considered, so only the key "a" is returned.

Example 4: Indexed Arrays

array_keys() can also be used with indexed arrays to retrieve the numeric indices.

<?php
$array = ["apple", "banana", "cherry"];

$keys = array_keys($array);
print_r($keys);
?>

Output:

Array
(
    [0] => 0
    [1] => 1
    [2] => 2
)

In this case, the function returns the indices (0, 1, 2) of the indexed array.

Practical Usage:

• array_keys() is useful for:
  • Extracting all keys from an associative array, especially when you need to iterate over keys instead of values.
  • Filtering keys based on a specific value, allowing you to quickly find the keys associated with that value.
  • Handling scenarios where you need to work with arrays of keys for further operations (e.g., inverting keys and values).

Summary:

• array_keys($array) returns an array containing all the keys from the input array.
• If a specific $value is provided, only keys associated with that value are returned.
• The function supports strict comparison (===) to compare both the value and its type.
• It works for both associative arrays and indexed arrays.

This function is versatile for extracting and filtering keys, making it a useful tool when working with arrays in PHP.