根据文档laravel5.5的passport版本是4.0
composer require laravel/passport=~4.0
这边遇到一个问题，random_compat 版本被锁定为 9.99.99
Installation request for paragonie/random_compat (locked at v9.99.99) -> satisfiable by paragonie/random_compat[v9.99.99]
解决方法，更新 random_compat 版本为～2.0
composer require paragonie/random_compat=~2.0