<?php
namespace SomeNamespace;
class BankAccount
{
    use BankAccountTrait;
}

trait BankAccountTrait {
    protected $balance = 0;

    public function getBalance()
    {
        return $this->balance;
    }

    protected function setBalance($balance)
    {
        if ($balance >= 0) {
            $this->balance = $balance;
        } else {
            throw new \RuntimeException;
        }
    }

    public function depositMoney($balance)
    {
        $this->setBalance($this->getBalance() + $balance);

        return $this->getBalance();
    }

    public function withdrawMoney($balance)
    {
        $this->setBalance($this->getBalance() - $balance);

        return $this->getBalance();
    }
}