我用的是 windows 操作系统。我也尝试了这些可用的解决方案：link 1）PHP pcntl模块安装link 2）如何在Windows中安装PCNTL扩展？

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Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - laravel/horizon v4.3.3 requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
    - laravel/horizon v4.3.2 requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
    - laravel/horizon v4.3.1 requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
    - laravel/horizon v4.3.0 requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
    - laravel/horizon 4.x-dev requires ext-pcntl * -> the requested PHP extension pcntl is missing from your system.
    - Installation request for laravel/horizon ^4.3 -> satisfiable by laravel/horizon[4.x-dev, v4.3.0, v4.3.1, v4.3.2, v4.3.3].

  To enable extensions, verify that they are enabled in your .ini files:
    - C:\laragon\bin\php\php-7.2.11-Win32-VC15-x64\php.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

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