PHP Projects · Lesson 2 of 8

PHP Calculator

Build a calculator from validated operands and an allowlisted operation.

Lesson

A calculator should parse numeric inputs and allow only known operations. Guard division by zero and distinguish invalid input from a valid result of zero. Avoid eval: it executes arbitrary PHP and turns a calculator into a code-execution vulnerability. The switch keeps each supported operation explicit. Further guidance Turn the feature into a sequence of small vertical slices: input, validation, domain behavior, persistence, response, and tests. Write acceptance criteria before implementation, protect each operation with authorization, and verify invalid input and failure cases. Document setup and keep deployment secrets out of the repository. Practice task: Create a fresh example for PHP Calculator. Predict the result before running it, then change one input and explain why the result changes. Test a normal case and one boundary or invalid case where it applies. Compare your output with the example and note one rule you would use in a real project.

Code examples

PHP Calculator

PHP

$a = 12;
$b = 4;
$operation = 'divide';
$result = match ($operation) {
    'add' => $a + $b,
    'divide' => $b !== 0 ? $a / $b : throw new DomainException('Division by zero'),
    default => throw new InvalidArgumentException('Unsupported operation'),
};
echo $result;

Expected output

Key points

Build a calculator from validated operands and an allowlisted operation.
Build and verify one complete feature at a time.
Test invalid input, permissions, and failure paths before deployment.

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