执行composer install后报以下错误：
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Your requirements could not be resolved to an installable set of packages.
Problem 1
- laravel/horizon v1.4.3 requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
- laravel/horizon v1.4.3 requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
- Installation request for laravel/horizon v1.4.3 -> satisfiable by laravel/horizon[v1.4.3].
解决办法：
rm -f composer.lock
再次执行composer install