Calculate Interest

<!--Write a PHP script CalculateInterest.php which generates an HTML page that contains:
An input text field to hold the amount of money; Radio buttons to choose the currency;
An input text field to enter the compound annual interest amount;
A dropdown menu to choose the period of time; A submit button.
When the information is submitted, the script should print out the amount of money you will have after the selected period, rounded to 2 decimal places. Semantic HTML is required. Styling is not required.-->

<!DOCTYPE html>
<html>
<head lang="en">
    <meta content="text/html" charset="UTF-8">
    <title>Calculate Interest</title>
</head>
<body>
<form action="#" method="post">
    <p>
        <label for="startAmount">Enter Amount</label>
        <input type="text" name="startAmount" id="startAmount" autofocus required/>
    </p>

    <p>
        <input type="radio" name="currency" id="currency1" value="0" checked/><label
            for="currency1">USD</label>
        <input type="radio" name="currency" id="currency2" value="1"/><label
            for="currency2">EUR</label>
        <input type="radio" name="currency" id="currency3" value="2"/><label
            for="currency3">BGN</label>
    </p>

    <p>
        <label for="compound-interest">Compound Interest Amount</label>
        <input type="text" name="compound-interest" id="compound-interest"/>
    </p>
    <select name="periodOfTime" id="dropdown-list">
        <option value="0">3 Months</option>
        <option value="1">6 Months</option>
        <option value="2">1 Year</option>
        <option value="3">2 Years</option>
        <option value="4">3 Years</option>
        <option value="5">5 Years</option>
    </select>
    <input type="submit" value="Calculate"/>
    <?php
    mb_internal_encoding('UTF-8');

    if ($_POST && array_key_exists('startAmount', $_POST) && array_key_exists('currency', $_POST) &&
        array_key_exists('compound-interest', $_POST) && array_key_exists('periodOfTime', $_POST)
    ) {
        if (!preg_match("/^\d+\.?\d{0,2}$/", trim($_POST['startAmount']))) {
            exit('Wrong input!!! - The Amount must been integer or float-point number with 2 digits after decimal
            point!');
        }

        if (!preg_match("/^\d+\.?\d*$/", trim($_POST['compound-interest']))) {
            exit('Wrong input!!! - The Compound Interest Amount must been integer or float-point number!');
        }

        $amount = (float)$_POST['startAmount'];
        $currencyTypes = array('&#36;', '&euro;', 'лв.');
        $currency = (array_key_exists($_POST['currency'], $currencyTypes)) ? $currencyTypes[$_POST['currency']] :
            null;
        $compoundInterest = (float)$_POST['compound-interest'];
        $months = array(3, 6, 12, 24, 36, 60);
        $timePeriod = (array_key_exists($_POST['periodOfTime'], $months)) ? $months[$_POST['periodOfTime']] :
            null;
        if (!$currency || !$timePeriod) {
            exit('Unexpected error!!! - Reload the Web Site again and carefully enter input fields!');
        }

        $compoundInterest = (($compoundInterest / 12) + 100) / 100;
        for ($i = 0; $i < $timePeriod; $i++) {
            $amount *= $compoundInterest;
        }

        $amount = round($amount, 2);
        echo ($_POST['currency'] == 2) ? $amount . ' ' . $currency : $currency . ' ' . $amount;
    }
    ?>
</form>
</body>
</html>