12/12/2016 - 4:26 AM




init – create the stack.
push – add an item to the top of the stack.
pop – remove the last item added to the top of the stack.
top – look at the item on the top of the stack without removing it.
isEmpty – return whether the stack contains no more items.

class ReadingList
    protected $stack;
    protected $limit;
    public function __construct($limit = 10) {
        // initialize the stack
        $this->stack = array();
        // stack can only contain this many items
        $this->limit = $limit;

    public function push($item) {
        // trap for stack overflow
        if (count($this->stack) < $this->limit) {
            // prepend item to the start of the array
            array_unshift($this->stack, $item);
        } else {
            throw new RunTimeException('Stack is full!'); 

    public function pop() {
        if ($this->isEmpty()) {
            // trap for stack underflow
	      throw new RunTimeException('Stack is empty!');
	  } else {
            // pop item from the start of the array
            return array_shift($this->stack);

    public function top() {
        return current($this->stack);

    public function isEmpty() {
        return empty($this->stack);

// DEMO:

$myBooks = new ReadingList();

$myBooks->push('A Dream of Spring');
$myBooks->push('The Winds of Winter');
$myBooks->push('A Dance with Dragons');
$myBooks->push('A Feast for Crows');
$myBooks->push('A Storm of Swords'); 
$myBooks->push('A Clash of Kings');
$myBooks->push('A Game of Thrones');

// Remove from stack:
echo $myBooks->pop(); // outputs 'A Game of Thrones'
echo $myBooks->pop(); // outputs 'A Clash of Kings'
echo $myBooks->pop(); // outputs 'A Storm of Swords'

// See whats at the top of the stack:
echo $myBooks->top(); // outputs 'A Feast for Crows'