Understanding PHP's array_key_first() Function: Syntax, Usage, and Examples

In PHP 7.3, a new array function array_key_first() was introduced, which returns the first key of an array.

Syntax

array_key_first (array $array) : mixed

Description

The function accepts an array and returns the value of its first key, or null if the array is empty.

Example 1:

$arr = ['a' => 1, 'b' => 2, 'c' => 3];

echo array_key_first($arr); // outputs a

Example 2:

$arr = [];

echo array_key_first($arr); // outputs null

Use Cases

1. Retrieve the first element's key name

Before PHP 7.3, developers often used reset() to get the first value and key() to get its key. Using array_key_first() is simpler.

$arr = ['a' => 1, 'b' => 2, 'c' => 3];

echo array_key_first($arr); // outputs a

2. Determine whether an array is empty

Previously empty() or count() were used. array_key_first() can check emptiness more directly.

$arr = [];

if (array_key_first($arr) === null) {
    echo 'Array is empty';
}

Result:

Array is empty

Note that if an array contains an element with a null value, array_key_first() may produce unexpected results.

Conclusion

The array_key_first() function, added in PHP 7.3, provides a convenient way to obtain the first key of an array and can also be used to check if an array is empty, but developers should be cautious when the array includes null values.