find the variance of all nodes in a linked list in php

main.php
class Node {
    public $data;
    public $next;
    
    function __construct($data) {
        $this->data = $data;
        $this->next = NULL;
    }
}

class LinkedList {
    public $head;
    
    function __construct() {
        $this->head = NULL;
    }
    
    function push($new_data) {
        $new_node = new Node($new_data);
        $new_node->next = $this->head;
        $this->head = $new_node;
    }
    
    function variance() {
        if ($this->head == NULL)
            return;
 
        $mean = 0;
        $size = 0;
        $current = $this->head;
        
        while ($current != NULL) {
            $mean += $current->data;
            $size++;
            $current = $current->next;
        }
        
        $mean /= $size;
 
        $variance = 0;
        $current = $this->head;
        
        while ($current != NULL) {
            $variance += pow(($current->data - $mean), 2);
            $current = $current->next;
        }
        
        return $variance / $size;
    }
}

//Example Usage
$llist = new LinkedList();
$llist->push(10);
$llist->push(20);
$llist->push(30);
$llist->push(40);

echo 'Variance of the linked list is: ' . $llist->variance();
1193 chars
60 lines

main.php
Variance of the linked list is: 125
36 chars
2 lines