Testing Exceptions with PHPUnit

PHPUnit's expectException() verifies that invalid inputs cause a specific exception rather than silently passing.

What It Does

Use expectException before calling the operation expected to fail. Check exception type relationships. Test invariants and edge cases, not just one happy path. Check exception type relationships.

Requirements and Compatibility

Use a supported PHP runtime and install any third-party tool or extension explicitly shown in the examples. Commands should be run from the project root unless stated otherwise.

Syntax, Inputs and Results

The examples separate configuration, the smallest working operation and a realistic follow-up task. Adapt file paths and dependency versions to your own project.

Example 1: Basic Use

Use expectException before calling the operation expected to fail. Check exception type relationships.

<?php
use PHPUnit\Framework\TestCase;
final class QuantityTest extends TestCase {
    public function testNegativeQuantityIsRejected(): void {
        $validate = static function (int $n): void {
            if ($n < 0) throw new InvalidArgumentException('Negative quantity');
        };
        $this->expectException(InvalidArgumentException::class);
        $validate(-2);
    }

    public function test1ArticleBoundary(): void { self::assertTrue(is_subclass_of(InvalidArgumentException::class, Throwable::class)); }
}

Example 2: Another Case

Test invariants and edge cases, not just one happy path. Check exception type relationships.

<?php
use PHPUnit\Framework\TestCase;
final class DiscountTest extends TestCase {
    public function testDiscountNeverExceedsPrice(): void {
        $discount = static fn (int $price, int $off): int => max(0, $price - $off);
        self::assertSame(0, $discount(30, 40));
        self::assertGreaterThanOrEqual(0, $discount(100, 5));
    }

    public function test2ArticleBoundary(): void { self::assertTrue(is_subclass_of(InvalidArgumentException::class, Throwable::class)); }
}

Practical Example

Target a failing test during debugging, then run the full suite.

vendor/bin/phpunit --filter testTaxIsAdded
vendor/bin/phpunit --stop-on-failure

For production, apply the compatibility and edge-case limits described below.

Important Behavior and Edge Cases

Separate isolated unit tests from integration tests requiring a real database or network.

Comparison with Related Functions and Tools

Subject	Key difference
Unit tests vs integration tests	Unit tests isolate one behavior; integration tests use actual interfaces, databases or framework components.

Common Beginner Mistakes

Mistake	Correct approach
Writing tests without meaningful assertions	Assert an exact result or expected exception, including at least one boundary case.
Skipping the prerequisite of this specific workflow	Install or configure the named tool and confirm the example can actually run.
Ignoring edge conditions	Separate isolated unit tests from integration tests requiring a real database or network.

Key Takeaways

PHPUnit's expectException() verifies that invalid inputs cause a specific exception rather than silently passing.
Separate isolated unit tests from integration tests requiring a real database or network.
Keep application configuration, source code and validation tests under version control.

Use these examples as a starting point, then validate the result with the actual PHP build, dependencies and inputs used by your application.