PHP SplDoublyLinkedList::unshift() Method: Usage and Examples

Prepends an entry at the start of the list.

The examples focus on the operation itself, including the important distinction: push appends a value at the end.

What It Does

Use the examples below to observe the behavior on a small dataset before extending it to application code.

Syntax and Return Value

SplDoublyLinkedList::unshift(mixed $value)

The displayed signature describes the API or identifies any correction or optional dependency. When a parameter is optional, supply it only when the task requires it.

Parameters

• value: required.

Example 1: Basic Use

The list begins with 3 values. Notice how unshift changes or observes its indexed deque rather than an ordinary associative array.

<?php
$list = new SplDoublyLinkedList();
$list->push('tea');
$list->push('coffee');
$list->push('water');
$list->unshift('first');
echo json_encode(iterator_to_array($list));

Output from a successful local PHP 8.4 run:

["first","tea","coffee","water"]

Example 2: Checking Behavior

The list begins with 4 values. Notice how unshift changes or observes its indexed deque rather than an ordinary associative array.

<?php
$list = new SplDoublyLinkedList();
$list->push('one');
$list->push('two');
$list->push('three');
$list->push('four');
$list->unshift('zero');
echo json_encode(iterator_to_array($list));

Output from a successful local PHP 8.4 run:

["zero","one","two","three","four"]

Example 3: Practical Example

The list begins with 3 values. Notice how unshift changes or observes its indexed deque rather than an ordinary associative array.

<?php
$list = new SplDoublyLinkedList();
$list->push(4);
$list->push(8);
$list->push(12);
$list->unshift(0);
echo json_encode(iterator_to_array($list));

Output from a successful local PHP 8.4 run:

[0,4,8,12]

Important Behavior and Edge Cases

Existing positions shift by one when a value is prepended.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Prepends an entry at the start of the list.
• Existing positions shift by one when a value is prepended.
• push appends a value at the end.

Use SplDoublyLinkedList::unshift when its documented behavior matches the problem. Test the relevant edge cases in your own runtime before deploying it.