从当前位置获取数组中的上一个和下一个键/值（PHP）

Question

I have an array similar to the following:

const BookIndex = array
(
    '1' => 'Chapter 1',
    '1.1' => 'Chapter 1.1',
    '1.1.1' => 'Chapter 1.1.1',
    '2' => 'Chapter 2',
    '2.1' => 'Chapter 2.1',
    '2.1.1' => 'Chapter 2.1.1',
);

Let's say that I have determined somehow that the current key (position) I care about is the '2' key. How do I find the previous and next keys?

$CurrentKey = '2';
$CurrentValue = BookIndex[$CurrentKey];

$PreviousKey = null; // I need to figure out the previous key from the current key.
$PreviousValue = BookIndex[$PreviousKey];

$NextKey = null; // I need to figure out the next key from the current key.
$NextValue = BookIndex[$NextKey];

Answer 1

You can use array functions for that

$NextKey = next($BookIndex); // next key of array

$PreviousKey = prev($BookIndex); // previous key of array

$CurrentKey = current($BookIndex); // current key of array

pointing to specific position

$CurrentKey = '2';

while (key($BookIndex) !== $CurrentKey) next($BookIndex);