PHP calculator

calculator.php

<?php

    const numberAInputName = "numberA";
    const numberBInputName = "numberB";
    const operationInputName = "operation";
    
    $operations = [
        "add" => function($a, $b) { return $a + $b;},
        "substract" => function($a, $b) { return $a - $b;},
        "multiply" => function($a, $b) { return $a * $b;},
        "divide" => function($a, $b) { return $a / $b;},
    ];

    $result = null;
    $error = null;
    $numberA = null;
    $numberB = null;
    $operation = null;
    $showResult = false;

    if(isset($_GET[numberAInputName])) {
        $numberA = $_GET[numberAInputName];
    }
    if(isset($_GET[numberBInputName])) {
        $numberB = $_GET[numberBInputName];
    }
    if(isset($_GET[operationInputName])) {
        $operation = $_GET[operationInputName];
    }

    function calculate(int $a, int $b, string $operation = "add") {
        global $operations;

        return $operations[$operation]($a,$b);
    } 

    if($numberA != null && $numberB != null) {
        try {
            $showResult = false;
            $result = calculate($numberA, $numberB, $operation);
            $showResult = true;
            $error = null;
        }
        catch(DivisionByZeroError $ex) {
            $error = "You can't divide by zero (0)!";
        }
    }

?>

<div>
    <form action="">
        <input type="number" name="numberA" value="<?php echo $numberA ?? 0; ?>">
        <select name="operation">
        <?php
            foreach($operations as $operationKey=>$operationValue):
        ?>
            <option value="<?php echo $operationKey; ?>"><?php echo $operationKey; ?></option>
            <?php endforeach; ?>    
        </select>
        <input type="number" name="numberB" value="<?php echo $numberB ?? 0; ?>">
        <button type="submit">Calculate</button>
    </form>

    <?php if($showResult): ?>
        <div style="background:#003366; color:white; font-size:2rem;text-align:center;"><?php echo $result; ?></div>
    <?php endif; ?>

    <?php if($error != null): ?>
        <div style="background:red; color:white; font-size:2rem;"><?php echo $error; ?></div>
    <?php endif; ?>
</div>