PHP SPL: Stack and Queue Tutorial

1. What is SPL?

SPL (Standard PHP Library) is a collection of built-in data structures and interfaces designed to solve common problems efficiently.

Why use SPL?

• Better performance than custom arrays
• Clear intent (stack vs queue)
• Cleaner, more readable code

We’ll focus on:

• SplStack
• SplQueue

---

2. Stack vs Queue (Concept)

Stack (LIFO)

• Last In, First Out

• Examples:

  • Undo/Redo
  • Function call stack
  • Browser back button

Queue (FIFO)

• First In, First Out

• Examples:

  • Job processing
  • Task scheduling
  • Message queues

---

3. SplStack

3.1 Creating a Stack

$stack = new SplStack();

---

3.2 Pushing Elements (push)

$stack->push("A");
$stack->push("B");
$stack->push("C");

Stack state:

Top -> C
       B
Bottom-> A

---

3.3 Popping Elements (pop)

echo $stack->pop(); // C

Explanation:

• Removes and returns the top element.

---

3.4 Peeking Top Element (top)

echo $stack->top(); // B

• Reads top element without removing it.

---

3.5 Checking if Stack is Empty

if ($stack->isEmpty()) {
    echo "Stack is empty";
}

---

3.6 Counting Elements

echo $stack->count(); // number of elements

---

3.7 Iterating a Stack

Important:

• SplStack iterates from top to bottom by default.

foreach ($stack as $item) {
    echo $item . PHP_EOL;
}

---

3.8 Real-World Example: Undo Feature

$undoStack = new SplStack();

$undoStack->push("Typed 'Hello'");
$undoStack->push("Typed 'World'");
$undoStack->push("Deleted 'World'");

// User presses undo
echo $undoStack->pop(); // Deleted 'World'

---

4. SplQueue

4.1 Creating a Queue

$queue = new SplQueue();

---

4.2 Enqueue (add to queue)

$queue->enqueue("Job1");
$queue->enqueue("Job2");
$queue->enqueue("Job3");

Queue state:

Front -> Job1, Job2, Job3 -> Back

---

4.3 Dequeue (remove from queue)

echo $queue->dequeue(); // Job1

• Removes and returns the first element.

---

4.4 Peeking Front Element

echo $queue->bottom(); // Job2

Explanation:

• bottom() → front of the queue
• top() → back of the queue

---

4.5 Checking if Queue is Empty

if ($queue->isEmpty()) {
    echo "Queue is empty";
}

---

4.6 Counting Elements

echo $queue->count();

---

4.7 Iterating a Queue

foreach ($queue as $job) {
    echo $job . PHP_EOL;
}

Iteration order:

• Front → Back

---

4.8 Real-World Example: Job Processing

$jobQueue = new SplQueue();

// Add jobs
$jobQueue->enqueue("Send Email");
$jobQueue->enqueue("Generate Invoice");
$jobQueue->enqueue("Notify User");

// Process jobs
while (!$jobQueue->isEmpty()) {
    $job = $jobQueue->dequeue();
    echo "Processing: $job" . PHP_EOL;
}

---

5. Stack vs Queue Summary

Feature	SplStack	SplQueue
Order	LIFO	FIFO
Add	push()	enqueue()
Remove	pop()	dequeue()
Typical Use	Undo, recursion	Jobs, tasks

---

6. Common Mistakes

❌ Using arrays instead of SPL

array_push($stack, $value); // Not recommended

Why?

• Slower
• Less expressive
• Easy to misuse order

Use SPL instead.

---

❌ Confusing top() and bottom() in Queue

• bottom() → front
• top() → back

---

7. When to Use SPL in Laravel / PHP Apps

Use SplStack when:

• Undo/redo
• Nested operations
• Temporary reversal logic

Use SplQueue when:

• Sequential jobs
• Rate-limited API calls
• Background task pipelines

Laravel internal usage:

• Symfony (Laravel’s base) uses SPL structures heavily.

---

8. Performance Note

• SPL structures are implemented in C
• Faster than PHP arrays for stack/queue operations
• Less memory overhead for frequent push/pop

---

9. Quick Cheat Sheet

// Stack
$stack->push($item);
$stack->pop();
$stack->top();

// Queue
$queue->enqueue($item);
$queue->dequeue();
$queue->bottom();