執行composer install遇到錯誤：Your requirements could not be resolved to an installable set of packages. 這是因為不匹配composer.json要求的版本。
$ composer install
Your requirements could not be resolved to an installable set of packages.
Problem 1
- laravel-notification-channels/webpush 3.0.0 requires minishlink/web-push ^2.0 -> satisfiable by minishlink/web-push[v2.0.0, v2.0.1].
- laravel-notification-channels/webpush 3.0.1 requires minishlink/web-push ^2.0 -> satisfiable by minishlink/web-push[v2.0.0, v2.0.1].
- minishlink/web-push v2.0.0 requires mdanter/ecc ^0.5.0 -> satisfiable by mdanter/ecc[v0.5.0, v0.5.1, v0.5.2].
- minishlink/web-push v2.0.1 requires mdanter/ecc ^0.5.0 -> satisfiable by mdanter/ecc[v0.5.0, v0.5.1, v0.5.2].
- mdanter/ecc v0.5.2 requires ext-gmp * -> the requested PHP extension gmp is missing from your system.
- mdanter/ecc v0.5.1 requires ext-gmp * -> the requested PHP extension gmp is missing from your system.
- mdanter/ecc v0.5.0 requires ext-gmp * -> the requested PHP extension gmp is missing from your system.
- Installation request for laravel-notification-channels/webpush ^3.0 -> satisfiable by laravel-notification-channels/webpush[3.0.0, 3.0.1].
To enable extensions, verify that they are enabled in your .ini files:
- /usr/local/php7.2.9/etc/php.ini
You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.
提示我的PHP 7版本太高，不符合composer.json需要的版本，但是在PHP 7下應該也是可以運行的，composer可以設置忽略版本匹配，命令是：
$ composer install --ignore-platform-reqs
或者
$ composer update --ignore-platform-reqs