A way to provide a test double for WordPress globals, like $wpdb, in order to unit test functions that use the global.

gistfile1.php

<?php

class TestDouble {
  private $stubs;
	
	public function stub( $method_name, $return_value ) {
		$this->stubs[ $method_name ] = $return_value;
	}

	public function __call( $method_name, $args ) {
		if ( isset( $this->stubs[ $method_name ] ) ) {
			return $this->stubs[ $method_name ];
		}
	}

	public function __get( $property_name ) {
	}
}

$wpdb = new TestDouble();

require_once('simpletest/autorun.php');

class WPTest extends UnitTestCase {
	function test_db_key_is_a_valid_key() {
		global $wpdb;
		// returns true when the key is in the database
		$wpdb->stub(
			$method = 'get_var',
			$will_return = '2'
		);
		$this->assertTrue( db_key_is_a_valid_key( 'invalid_key' ) );

		//returns false when the key is not in the database
		$wpdb->stub(
			$method = 'get_var',
			$will_return = null
		);
		$this->assertFalse( db_key_is_a_valid_key( 'valid_key' ) );
	}
}

function db_key_is_a_valid_key( $key ) {
	global $wpdb;
	$id_of_site_record_belonging_to_provided_home_url = $wpdb->get_var( $wpdb->prepare(
		"SELECT ID FROM " . $wpdb->prefix . "sites WHERE site_key = %s",
		$key
	) );

	return ! empty( $id_of_site_record_belonging_to_provided_home_url );
}

Here I'm unit testing the db_key_is_a_valid_key function.

It is a boolean function, which returns whether a provided key is a valid one or not.

Whether the function returns true or false is dependent on the return value of the $wpdb->get_var function, so that return value needs to be stubbed out to stimulate each possible case.

The get_var method of the wpdb class returns

• null if for whatever reason the DB query does not return any rows, including if the query is malformed.
• a single value if the query returns rows