How to mock MySQLi when unit testing with PHPUnit

PHPUnit is the most used unit testing framework for PHP. Today I wanted to unit test some PHP code that relies on MySQLi. The dilemma is that you either need an actual database and load a fixture or you need to mock the database. As Claudio Lasalla clearly puts:

Unit tests are not “unit” tests if they test things other than the System Under Test (SUT).

And further explains:

Unit tests check on the behavior of units. Think of a class as being a unit. Classes, more often than not, have external dependencies. Tests for such classes should not use their real dependencies because if the dependencies have defects, the tests fail, even though the code inside the class may be perfectly fine.

This theory made total sense to me. That’s why I decided to mock the MySQLi dependency. In this post I will show you just how far I came before I realized this was not going to work out (for me).

The code

The test class, that extends “PHPUnit Framework TestCase”, has an extra method “expectQueries()”. The class looks like this:


class MySQL_CRUD_API_Test extends PHPUnit_Framework_TestCase
	private function expectQueries($queries)
		$mysqli = $this->getMockBuilder('mysqli')
			->will($this->returnCallback(function($str) { return addslashes($str); }));
			->will($this->returnCallback(function($query) use ($queries) {
				$results = $queries[$query];
				$mysqli_result = $this->getMockBuilder('mysqli_result')
					->will($this->returnCallback(function() use ($results) {
						static $r = 0;
						return isset($results[$r])?$results[$r++]:false;
				return $mysqli_result;

		return $mysqli;

	public function testSomeSubjectThatUsesMysqli()
		$mysqli = $this->expectQueries(array(
			"SELECT * FROM `table`" =>array(array('1','value1'),array('2','value2'),array('3','value3')),
			"SELECT * FROM `table` LIMIT 2" =>array(array('1','value1'),array('2','value2')),
			// other queries that may be called
		// do something that uses $mysqli

The subject-under-test is actually doing something like this:

$result = $mysqli->query("SELECT * FROM `table`");
while ($row = $result->fetch_row()) {
	// do something with the data in $row

And in the test it will return the corresponding rows for the queries that you execute. Nice huh?

Not ready

This is a proof-of-concept of a mock of the MySQLi component for PHPUnit. The ‘real_escape_string’ function has a sloppy implementation. It does not (yet) support the much used ‘prepare’, ‘execute’ or ‘fetch_fields’ methods. To give an idea of the completeness, for MySQLi it now support 2/62 functions and properties, for MySQLi Statement 0/28 and for MySQLi Result 2/15. Apart from this incompleteness there is the problem that you may need to support meta information, such as field names and types, to have a fully working mock. If you feel like continuing my work, then feel free to take my code.


Although this was a nice exercise and it may even be the right thing to do in theory, it did not seem to make much sense (to me) in practice. So I gave up on this approach and my current implementation runs all tests against a real database. It loads a database from a SQL file (fixture) in the static ‘setUpBeforeClass()’ function. This may not be so ‘correct’ or ‘clean’ (from a unit testing point of view), but it is much faster to write and easier to maintain.

My question for you: Am I wrong or is the theory wrong? Please tell me using the comments.


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