Mocking a static method call

example.php

<?php

// Before.
class Db {
    public static function query() {
        // Do the request.
        return $result;
    }
}

// After.
class Db {
    public static function query($sql, $params) {
        if (!self::$instance) {
            self::$instance = new NonStaticDb();
        }
        return self::$instance->query($sql, $params);
    }

    public static function mockWith($instance) {
        self::$instance = $instance;
    }
}

class NonStaticDb {
    public function query() {
        // Do the request.
        return $result;
    }
}