/bootstrap/start.php是在composer install之后通过运行Laravel的php artisan optimize创建的.在Laravel升级过程中，我遇到了很多问题，但是删除/bootstrap/start.php，composer.lock和vendor目录并重新运行composer install应该可以解决此问题.

/bootstrap/start.php is created after composer install by running Laravel's php artisan optimize. I've had a lot of issues on this during upgrades of Laravel, but removing /bootstrap/start.php, composer.lock, and the vendor directory and re-running composer install should fix this issue.