PHP Testing with PHPUnit

Before writing code

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PHPUnit Fundamentals

Test Class Structure

• Extend PHPUnit\Framework\TestCase (or WC_Unit_Test_Case for WooCommerce)
• Test methods prefixed with test or annotated with #[Test]
• setUp() — runs before each test
• tearDown() — runs after each test
• setUpBeforeClass() / tearDownAfterClass() — run once per class

Assertions

Core assertions:

• assertEquals( $expected, $actual ) — loose equality
• assertSame( $expected, $actual ) — strict identity
• assertTrue( $condition ) / assertFalse( $condition )
• assertNull( $value ) / assertNotNull( $value )
• assertCount( $expected, $array )
• assertContains( $needle, $haystack )
• assertInstanceOf( $class, $object )
• assertArrayHasKey( $key, $array )
• assertStringContainsString( $needle, $haystack )

Exception Testing

• $this->expectException( InvalidArgumentException::class )
• $this->expectExceptionMessage( 'Expected message' )
• $this->expectExceptionCode( 404 )

Test Doubles

Mocks

Create mock objects to verify interactions:

$mock = $this->createMock( PaymentProcessor::class );
$mock->expects( $this->once() )
     ->method( 'charge' )
     ->with( $this->equalTo( 29.99 ) )
     ->willReturn( true );

Stubs

Return predetermined values without verifying calls:

$stub = $this->createStub( PriceCalculator::class );
$stub->method( 'calculate' )->willReturn( 29.99 );

Expectation Methods

• $this->once() — called exactly once
• $this->exactly( $n ) — called exactly n times
• $this->never() — not called
• $this->atLeastOnce() — called one or more times
• $this->any() — any number of calls

willReturn Variants

• willReturn( $value ) — return fixed value
• willReturnMap( $map ) — return based on arguments
• willReturnCallback( $callable ) — custom logic
• willThrowException( $exception ) — throw on call

Data Providers

What They Do

Supply multiple test cases to a single test method:

public static function priceProvider(): array {
    return [
        'regular price' => [ 29.99, 29.99, '' ],
        'sale price'    => [ 29.99, 19.99, '19.99' ],
        'zero price'    => [ 0.00, 0.00, '' ],
    ];
}

#[DataProvider('priceProvider')]
public function test_get_display_price( float $regular, float $expected, string $sale ): void {
    // test with each data set
}

Best Practices for Providers

• Name data sets descriptively (keyed array)
• Keep providers as static methods
• Return arrays of arrays (each inner array = one test case)
• Use #[DataProvider('methodName')] attribute

WordPress/WooCommerce Testing Specifics

WP_UnitTestCase

• Provides WordPress-loaded environment
• Each test wrapped in a database transaction (auto-rollback)
• Factory methods: $this->factory->post, $this->factory->user
• go_to( $url ) — simulate page load
• set_current_screen( 'edit-post' ) — simulate admin screen

Mocking WordPress Functions

Use WP_Mock or Brain\Monkey libraries:

• Mock add_action, add_filter, do_action, apply_filters
• Mock wp_remote_get, get_option, etc.
• Useful for isolated unit tests without WordPress bootstrap

Mocking HTTP Requests

Filter pre_http_request to intercept wp_remote_get() / wp_remote_post():

add_filter( 'pre_http_request', function( $preempt, $args, $url ) {
    return [ 'response' => [ 'code' => 200 ], 'body' => '{"status":"ok"}' ];
}, 10, 3 );

Test Organization

Directory Structure

tests/
├── bootstrap.php           # Test bootstrap
├── Unit/                   # Pure unit tests (no WP)
│   └── PriceCalculatorTest.php
├── Integration/            # Tests with WP/WC loaded
│   ├── ProductTest.php
│   └── OrderTest.php
└── E2E/                    # Playwright end-to-end
    └── specs/

phpunit.xml.dist Configuration

• Define test suites: unit, integration
• Set bootstrap file
• Configure coverage report output
• Set environment variables

TDD Workflow

1. Red — write a failing test that describes the desired behavior
2. Green — write the minimum code to make the test pass
3. Refactor — clean up code while keeping tests green
4. Repeat

Best Practices

• One assertion per test (when practical)
• Name tests descriptively: test_process_payment_returns_success_for_valid_card
• Use data providers for parameterized tests
• Mock external dependencies (APIs, databases, file system)
• Test edge cases: null values, empty strings, boundary values, invalid input
• Keep unit tests fast — no network, no filesystem, no database
• Use integration tests for WordPress/WooCommerce interaction testing
• Run tests in CI on every push

Fetch the PHPUnit documentation for exact assertion signatures, mock API, and configuration options before implementing.