Queue Enqueue/Dequeue

Algorithm

The replay uses the same three values in every language, so this PHP DSA implementation can be compared directly with the rest of the DSA track.

front The front is the oldest value still waiting in the queue.

FIFO A queue removes values in first-in, first-out order.

Visual walkthrough

The queue keeps the oldest value at the front and adds new values at the back.

Step 1 - Enqueue 10, 20, 30

New values join at the back. The oldest value, 10, stays at the front.

Step 2 - Dequeue removes 10

Removing from the front returns 10 and makes 20 the new front.

Basic Implementation

                            basic.php
                        

                            <?php
function render_values(array $values): string { return implode(" -> ", $values); }
$queue = [];
foreach ([10, 20, 30] as $value) { $queue[] = $value; }
$removed = [];
while (count($queue) > 0) { $removed[] = array_shift($queue); }
echo render_values($removed) . PHP_EOL;

                        

Complexity

Time: O(1) per operation with a real queue
Space: O(n)

Implementation notes

$queue = [] starts as an empty PHP array, and the trace records that exact empty state first.
The enqueue loop is foreach ([10, 20, 30] as $value) { $queue[] = $value; }, so each value is appended at the back of the same array.
After enqueue, the replayed queue state is [10, 20, 30].
$removed = [] collects the dequeue order separately from the live queue.
The dequeue loop runs while count($queue) > 0 and uses array_shift($queue) to remove the front element.
In PHP, array_shift mutates the array by removing the first value; with these numeric positions, the remaining values slide forward in queue order.
That makes the source easy to replay for three pinned values, but it is not the scalable PHP queue shape; a larger queue would keep a head index or use a real queue container instead of shifting the array each time.
The first dequeue moves 10 into $removed, leaving $queue as [20, 30].
The remaining dequeues produce $removed = [10, 20, 30] and $queue = [].
render_values($removed) joins values with " -> ", and echo ... . PHP_EOL prints 10 -> 20 -> 30.