Calculator.php

<?php


//final calculator
class Calculator
{
    /**
     * The start value.
     *
     * @var null
     */
    protected $result = null;

    /**
     * The array of operands.
     *
     * @var null
     */
    protected $operands = [];

    /**
     * The operation class name.
     *
     * @var null
     */
    protected $operation;

    public function setResult($num)
    {
        $this->result = $num;
    }

    /**
     * Get the result of the operation
     *
     * @return int
     */
    public function getResult()
    {
        return $this->result;
    }

    /**
     * We set the operands array equals to the arguments given.
     */
    public function setOperands()
    {
        $this->operands = func_get_args();
    }

    /**
     * We inject the Operation class.
     */
    public function setOperation(Operation $operation)
    {
        $this->operation = $operation;
    }

    /**
    * This is the core function.
    * It loop throught the operands.
    * Foreach operands it checks if it's numeric then call the run method of the 
    * operation class. 
    * After the loop we return the final result.
    */
    public function calculate()
    {
        foreach ($this->operands as $num)
        {
            if (!is_numeric($num)){
                throw new InvalidArgumentException;
            }

            $this->result = $this->operation->run($num, $this->result);
        }
        return $this->result;
    }
}