服务器上装了两个php版本：

$ php -v
PHP 7.4.13 (cli) (built: Nov 28 2020 06:24:59) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
    with Zend OPcache v7.4.13, Copyright (c), by Zend Technologies

以及：

$ php8.0 -v
PHP 8.0.0 (cli) (built: Nov 27 2020 12:26:22) ( NTS )
Copyright (c) The PHP Group
Zend Engine v4.0.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.0.0, Copyright (c), by Zend Technologies

因为两个php 装的扩展不同（composer默认使用php7.4的版本），比如php7.4的版本没有装curl,在执行composer update的时候会报错：

$ composer update

Composer is operating significantly slower than normal because you do not have the PHP curl extension enabled.
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires PHP extension ext-curl * but it is missing from your system. Install or enable PHP's curl extension.

To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php/7.4/cli/php.ini
    - /etc/php/7.4/cli/conf.d/10-mysqlnd.ini
    - /etc/php/7.4/cli/conf.d/10-opcache.ini
    - /etc/php/7.4/cli/conf.d/10-pdo.ini
    - /etc/php/7.4/cli/conf.d/15-xml.ini
    - /etc/php/7.4/cli/conf.d/20-calendar.ini
    - /etc/php/7.4/cli/conf.d/20-ctype.ini
    - /etc/php/7.4/cli/conf.d/20-dom.ini
    - /etc/php/7.4/cli/conf.d/20-exif.ini
    - /etc/php/7.4/cli/conf.d/20-ffi.ini
    - /etc/php/7.4/cli/conf.d/20-fileinfo.ini
    - /etc/php/7.4/cli/conf.d/20-ftp.ini
    - /etc/php/7.4/cli/conf.d/20-gd.ini
    - /etc/php/7.4/cli/conf.d/20-gettext.ini
    - /etc/php/7.4/cli/conf.d/20-iconv.ini
    - /etc/php/7.4/cli/conf.d/20-json.ini
    - /etc/php/7.4/cli/conf.d/20-mbstring.ini
    - /etc/php/7.4/cli/conf.d/20-mysqli.ini
    - /etc/php/7.4/cli/conf.d/20-pdo_mysql.ini
    - /etc/php/7.4/cli/conf.d/20-phar.ini
    - /etc/php/7.4/cli/conf.d/20-posix.ini
    - /etc/php/7.4/cli/conf.d/20-readline.ini
    - /etc/php/7.4/cli/conf.d/20-shmop.ini
    - /etc/php/7.4/cli/conf.d/20-simplexml.ini
    - /etc/php/7.4/cli/conf.d/20-sockets.ini
    - /etc/php/7.4/cli/conf.d/20-swoole.ini
    - /etc/php/7.4/cli/conf.d/20-sysvmsg.ini
    - /etc/php/7.4/cli/conf.d/20-sysvsem.ini
    - /etc/php/7.4/cli/conf.d/20-sysvshm.ini
    - /etc/php/7.4/cli/conf.d/20-tokenizer.ini
    - /etc/php/7.4/cli/conf.d/20-xmlreader.ini
    - /etc/php/7.4/cli/conf.d/20-xmlwriter.ini
    - /etc/php/7.4/cli/conf.d/20-xsl.ini
You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

所以这里就要用装了curl扩展的php8.0来执行，首先找到composer 的安装路径：

$ whereis composer
composer: /usr/local/bin/composer

然后执行：

php8.0 /usr/local/bin/composer update

此时composer使用的就是php8.0 这个版本啦~~~