It should be noted that next() sends an implicit null into the generator to ensure that code execution isn't blocked if the generator is expecting a value to be sent into it and the code doesn't execute any send()
(PHP 5 >= 5.5.0, PHP 7)
Generator::next — Resume execution of the generator
이 함수는 인수가 없습니다.
값을 반환하지 않습니다.
It should be noted that next() sends an implicit null into the generator to ensure that code execution isn't blocked if the generator is expecting a value to be sent into it and the code doesn't execute any send()