//index.php
<form action="" method="get">
    <input type="number" name="firstNumb" placeholder="Введите 1-е число" value="<?php echo $_GET['firstNumb'] ? $_GET['firstNumb'] : null ;?>" required>
    <select name="symbol">
        <?php foreach (['+','-','/','*'] as $key => $val) {
            if( $val ==  $_GET['symbol'] ){
                ?>
                    <option value='<?php echo $val; ?>' selected><?php echo $val; ?></option>
                <?php
            }else{
                ?>
                <option value='<?php echo $val; ?>'><?php echo $val; ?></option>
                <?php
            }
        } ?>
    </select><br>
    <input type="number" name="secondNumb" placeholder="Введите 2-е число" value="<?php echo $_GET['secondNumb'] ? $_GET['secondNumb'] : null ;?>" required>
    <input type="submit" name="submitNumb" value='='>
</form>

Результат: <?php echo calculate(); ?>

//functions.php
function calculate(){
    switch($_GET['symbol']){
        case '+':
            return $_GET['firstNumb'] + $_GET['secondNumb']; break;
        case '-':
            return $_GET['firstNumb'] - $_GET['secondNumb']; break;
        case '*':
            return $_GET['firstNumb'] * $_GET['secondNumb']; break;
        case '/':
            return $_GET['firstNumb'] / $_GET['secondNumb']; break;
        default:
            return $_GET['firstNumb'] + $_GET['secondNumb']; break;
    }
}