Stack Using Array in DSA – Implementation, Operations & PHP Code

📘 Stack Using Array

🚀 Introduction

A Stack is a linear data structure that follows:

👉 LIFO (Last In, First Out)

In this approach, we implement a stack using an array, where the last element represents the top of the stack.

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🔹 Stack Representation

Top → [30, 20, 10]

👉 Here:

• 30 is the top element
• 10 is the bottom element

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🔹 Basic Operations

1️⃣ Push

Adds an element to the top.

2️⃣ Pop

Removes the top element.

3️⃣ Peek

Returns the top element.

4️⃣ isEmpty

Checks if stack is empty.

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🔹 Algorithm

✔️ Push Operation

1. Check if stack is full (optional in dynamic array)
2. Add element to array
3. Update top

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✔️ Pop Operation

1. Check if stack is empty
2. Remove last element
3. Return value

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🧑‍💻 PHP Implementation

<?phpclass Stack {
    private $stack = [];
    private $size;    public function __construct($size = 10) {
        $this->size = $size;
    }    // Push operation
    public function push($value) {
        if (count($this->stack) >= $this->size) {
            return "Stack Overflow";
        }        $this->stack[] = $value;
        return "Inserted: $value";
    }    // Pop operation
    public function pop() {
        if (empty($this->stack)) {
            return "Stack Underflow";
        }        return array_pop($this->stack);
    }    // Peek operation
    public function peek() {
        if (empty($this->stack)) {
            return "Stack is Empty";
        }        return end($this->stack);
    }    // isEmpty
    public function isEmpty() {
        return empty($this->stack);
    }    // Display stack
    public function display() {
        echo "Stack: ";
        foreach (array_reverse($this->stack) as $val) {
            echo $val . " ";
        }
    }
}// Example
$stack = new Stack(5);echo $stack->push(10) . "\n";
echo $stack->push(20) . "\n";
echo $stack->push(30) . "\n";echo "Top: " . $stack->peek() . "\n";echo "Popped: " . $stack->pop() . "\n";$stack->display();?>

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⚡ Time Complexity

Operation	Complexity
Push	O(1)
Pop	O(1)
Peek	O(1)

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🎯 Advantages

✅ Easy to implement
✅ Fast operations (O(1))
✅ Uses built-in array features

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❌ Disadvantages

❌ Fixed size (if defined)
❌ Possible overflow
❌ Memory wastage

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🧠 Real-World Use Cases

• Undo/Redo operations
• Expression evaluation
• Backtracking algorithms
• Browser history

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🧠 Interview Tips

👉 Common questions:

• Implement stack without using built-in functions
• Two stacks in one array
• Min stack problem

👉 Important:
👉 Always handle:

• Stack Overflow
• Stack Underflow

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🏁 Conclusion

Implementing a stack using an array is the most basic and efficient approach.

👉 It helps you understand:

• LIFO principle
• Memory management
• Core stack operations