Mocking MySQLi in PHPUnit

TL;DR: If you need to mock public properties as well, wrap MySQLi and mock that wrapper instead. Check the code in the master branch for how to achieve that. PHP version at the time of writing: 7.

Long version:

Recently I was given a test task where a class took a container object as its dependency. I thought I was being clever switching it to mysqli_result so that I could read in the results one by one and yield objects from the data on the fly using generators. However, mocking mysqli objects turned out to be a bit more difficult than I expected.

If your code uses only methods and not any public properties of mysqli objects, mocking the objects works just fine. But if you try to set any of the known public properties for the mocks, e.g. $field_count or $num_rows in mysqli_result object, you'll run into trouble. This is because those are by design read-only properties.

I tried assigning the mysqli_result::$num_row property both via the Reflection API and by adding my own mocked magic-method getter to the mocked class, with little success with either method. I didn't take a deep look into why exactly the latter method fails but if you're using MySQLi extension and need to mock it for unit tests, I would recommend creating a wrapper for MySQLi and mocking the wrapper instead as an easier alternative.

The master branch of this repository uses a wrapper as dependency and mocks that in the unit tests. It's not a full wrapper around MySQLi but rather it only mocks the properties that the code uses, here fetch_array method and num_rows property. You'll have to create your own wrapper and mock for it but hopefully this'll give you an idea how to do it.

Running the tests

Requires PHP 7.

Clone the repository
Download Composer into the repository directory
Run the following in the repository directory:

./composer.phar install
vendor/bin/phpunit tests

Errors in mocking mysqli when properties are used

You can check out branch with-mysqli-dependency and modify the code in tests/app/MysqliMockTrait.php and src/services/UserService.php as instructed in the code's comments to reproduce the errors down below. Better yet if you can fix them!

With reflections

$reflection = new \ReflectionClass($mysqliResult);
$numRowsProperty = $reflection->getProperty('num_rows');
$numRowsProperty->setAccessible(true);
$numRowsProperty->setValue($mysqliResult, count($results));

Result:

PHP Fatal error: ReflectionProperty::setValue(): Cannot write property in /mysqli_mockup/tests/app/MysqliMockTrait.php

With __get override

$mysqliResult
    ->expects($this->any())
    ->method('__get')
    ->with($this->equalTo('num_rows'))
    ->will($this->returnValue(count($results)));