## crossJoin 方法交叉连接指定数组或集合的值，返回所有可能排列的笛卡尔积： ``` $collection = collect([1, 2]); $matrix = $collection->crossJoin(['a', 'b']); $matrix->all(); /* [ [1, 'a'], [1, 'b'], [2, 'a'], [2, 'b'], ] */ $collection = collect([1, 2]); $matrix = $collection->crossJoin(['a', 'b'], ['I', 'II']); $matrix->all(); /* [ [1, 'a', 'I'], [1, 'a', 'II'], [1, 'b', 'I'], [1, 'b', 'II'], [2, 'a', 'I'], [2, 'a', 'II'], [2, 'b', 'I'], [2, 'b', 'II'], ] */ ```