Interest rate calculator

rate.php

<?php

class Account
{
    private $capital;
    private $rate;
    
    public function __construct(float $capital, float $rate)
    {
        $this->capital = $capital;
        $this->rate = $rate;
    }
    
    public function transferIn(float $amount) : void
    {
        if ($amount < 0) {
            throw new Exception('Can\'t transfer negative amounts.');
        }
        $this->capital += $amount;
        
        return;
    }
    
    public function transferOut(Account $account, float $amount) : void
    {
        if ($amount < 0) {
            throw new Exception('Can\'t transfer negative amounts.');
        }
        
        if ($amount > $this->capital) {
            $amount = $this->capital;
        }
        
        $this->capital -= $amount;
        $account->transferIn($amount);
        
        return;
    }
    
    public function calculateAnnualInterest() : float
    {
        return ($this->capital/100)*$this->rate;
    }
    
    public function calculateMonthlyInterest() : float
    {
        return ($this->calculateAnnualInterest()/12);
    }
}

$interest = 0;
$account1 = new Account(9000, 2);
$account2 = new Account(0, 2.25);

for ($i = 1; $i <= 12; $i++) {
    $account1->transferOut($account2, 500);
    $interest1 = $account1->calculateMonthlyInterest();
    $interest2 = $account2->calculateMonthlyInterest();
    $monthlyInterest = $interest1+$interest2;
    $interest += $monthlyInterest;
    echo 'Total for month ' . $i . ': ' . $interest1 . ' + ' . $interest2 . ' = ' . $monthlyInterest . PHP_EOL;
}

echo 'Total: ' . $interest;