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環境：php7.1、Laravel5.5

composer update

実行したら

laravel/framework v5.5.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

これな

素直に

sudo yum install --enablerepo=remi-php71 php-mbstring

次のエラー

phpunit/phpunit 6.3.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.

またかよ

sudo yum install --enablerepo=remi-php71 php-xml

OK!!!