How to access the i-th position of an array?

Question

I have the following statement

1. Define a stored stack in an array; a. Insert a widget at the top of the stack (push); b. Removing an item from the top of the stack (pop); c. Return the number of stack elements (elementsNumbers);
2. Write functions for: a. Insert an element in the eleventh position of the stack; b. Remove an element from the ith stack position;

The whole item 1 I already finished in PHP with the STACK class however now is the monster, because how will I be able to access the ith position of the stack? which in this case is item 2

Code:

<?php

class Stack
{
    protected $stack;
    protected $size;
    
    public function __construct($size = 50) {
        
        //Initialize stack
        $this->stack = array();
        
        //Initialize stack size
        $this->size  = $size;
    }
    
    /**
     * Insert an element in a stack
     * @param type $data
     */
    public function push($data) {
        
        //Check stack overflow
        if(count($this->stack) < $this->size) {
            
            //Inserts an element at the beginning
            array_unshift($this->stack, $data);
            
        } else {
            
            // If stack is full then throw stack overflow exception
            throw new RuntimeException("Stack overflow");
            
        }
    }
    
     /**
     * Removes an element from a stack
     *
     */
    public function pop() {
        
        // If stack is empty
        if (empty($this->stack)) {
            
            throw new RuntimeException("Stack underflow");
            
        } else {
            
            return array_shift($this->stack);
        }
    }
    
   
}
 
$stack = new Stack();
 
$stack->push(4);
$stack->push(5);
 
echo $stack->pop();  // Pop 5
 
$stack->push(7);
$stack->push(9);
$stack->push(8);
 
echo $stack->pop();  // Pop 8
echo $stack->pop();  // Pop 9