public function testDiff() { $collection1 = new Collection(['a', 'b', 'e']); $collection2 = new Collection(['a', 'c']); $collection3 = new Collection(['d', 'e']); $result = $collection1->diff($collection2, $collection3); $this->assertNotSame($collection1, $result); $this->assertInstanceOf(get_class($collection1), $result); $this->assertSame([1 => 'b'], (array) $result); }