<?php
/**
 * Created by PhpStorm.
 * User: zambahacker
 * Date: 28/07/15
 * Time: 21.12
 */

class CercainArray{
    private $array;

    public function __construct($array){
        $this->array = $array;
    }

    public function trovaNum(){
        $array = count($this->array) + 1;
        $somma = 0;
        foreach($this->array as $numero){
            $somma += $numero;
        }
        return (($array*($array+1))/2) - $somma;
    }

}

$array = array(3, 4, 9, 2, 7, 1, 8, 6);

$cerca = new CercainArray($array);
echo $cerca->trovaNum();