<?php $y(); x1(); x2()(); x3()()(); x4()()()();
<?php $y1(1); $y2(1)(2); $y3(1)(2)(3); \x1(1); \x2\y2(2)(22); \x3(3)(33)(333); \x4\y4\z4(4)(44)(444)(4444);
<?php $y(1); $y(1, 2); x1(1, 2); x2(1, 2)(3, 4); x3(1, 2)(3, 4)(5, 6); x4(1, 2)(3, 4)(5, 6)(7, 8);