Como escrever essa classe de uma forma mais profissional?

<?php

class ContaCorrente {

    // private $creditos = 1200;
    // private $debitos = 650;

    public function MostraSaldo()
    {
        $mostraSaldo = $this->CalculaSaldo($this->creditos, $this->debitos);
        return $mostraSaldo;
    }

    public function CalculaSaldo($creditos, $debitos)
    {
        $this->creditos = $creditos;
        $this->debitos = $debitos;

        $saldo = $creditos - $debitos;

        return $saldo;
    }
}

###################################################
###################################################

$obj = new ContaCorrente;

$obj->CalculaSaldo(1200, 600);

echo $obj->MostraSaldo();