先说说基本情况，Magento2.3.4的系统，从线上git clone到本机搞开发环境，配置好域名后，打开域名就给我报了Vendor autoload is not found. Please run 'composer install' under application root directory.

很明显git clone下载的系统，并不会把Vendor目录等信息copy到我本机，这种情况下，可以直接用ftp下载站点代码来解决

但本着学习的心态，试试不同的安装方式，使用composer install来安装magento系统，入一下composer install安装的坑吧，记录一下所有安装过程遇到的问题及解决方案：

一、按提示进入网站根目录，执行composer install，结果报了以下错误

vice@srv /v/w/h/demo_m2_234 (master)> composer install
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
    - Installation request for amzn/amazon-pay-sdk-php 3.6.0 -> satisfiable by amzn/amazon-pay-sdk-php[3.6.0].
    - amzn/amazon-pay-sdk-php 3.6.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
    
****省略******
   
    - Installation request for amzn/amazon-pay-and-login-with-amazon-core-module 3.3.1 -> satisfiable by amzn/amazon-pay-and-login-with-amazon-core-module[3.3.1].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php/7.3/cli/php.ini
    - /etc/php/7.3/cli/conf.d/10-mysqlnd.ini
****省略****** 
    - /etc/php/7.3/cli/conf.d/20-sysvshm.ini
    - /etc/php/7.3/cli/conf.d/20-tokenizer.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

二：查看了一下根目录的文件，存在composer.lock文件，于是先重命名为composer.lock.bak，再尝试composer install，操作如下

vice@srv /v/w/h/demo_m2_234 (master)> mv composer.lock composer.lock.bak
vice@srv /v/w/h/demo_m2_234 (master)> composer install
Loading composer repositories with package information
Warning from repo.magento.com: You haven't provided your Magento authentication keys. For instructions, visit https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html
    Authentication required (repo.magento.com):
      Username:
提示我输入magento账户

三：输入magento登录的账号密码后，出现以下错误

Warning from repo.magento.com: Your Magento authentication keys are invalid. Please double-check your keys in your Marketplace account. For instructions, visit https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html
Warning from repo.magento.com: Your Magento authentication keys are invalid. Please double-check your keys in your Marketplace account. For instructions, visit https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html
Warning from repo.magento.com: Your Magento authentication keys are invalid. Please double-check your keys in your Marketplace account. For instructions, visit https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html

PHP Fatal error:  Uncaught Error: Call to undefined function Symfony\Component\Console\mb_detect_encoding() in /usr/share/php/Symfony/Component/Console/Application.php:1217
Stack trace:
#0 /usr/share/php/Symfony/Component/Console/Application.php(884): Symfony\Component\Console\Application->splitStringByWidth('Invalid credent...', 185)
#1 /usr/share/php/Symfony/Component/Console/Application.php(859): Symfony\Component\Console\Application->doActuallyRenderThrowable(Object(Composer\Downloader\TransportException), Object(Symfony\Component\Console\Output\StreamOutput))
#2 /usr/share/php/Symfony/Component/Console/Application.php(822): Symfony\Component\Console\Application->doRenderThrowable(Object(Composer\Downloader\TransportException), Object(Symfony\Component\Console\Output\StreamOutput))
#3 /usr/share/php/Symfony/Component/Console/Application.php(131): Symfony\Component\Console\Application->renderThrowable(Object(Composer\Downloader\TransportException), Object(Symfony\Component\Console\Output\StreamOutput))
#4 /usr/share/php/Sym in /usr/share/php/Symfony/Component/Console/Application.php on line 1217

关键的第一句Warning from repo.magento.com: Your Magento authentication keys are invalid. Please double-check your keys in your Marketplace account. For instructions, visit https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html，好吧，复制连接进入看了一下，再google查查错误信息。

异常原因：其实就是没有magento插件市场的key，注册magento用户登录后创建key，再重新用composer install进行安装，找到解决两个方案

三 (1）:把账号/密码换成magento的Public Key/Private Key，操作如下

vice@srv /v/w/h/demo_m2_234 (master) [SIGINT]> composer install -vvv
Reading ./composer.json
Loading config file ./composer.json
Checked CA file /etc/ssl/certs/ca-certificates.crt: valid
Failed to initialize global composer: Composer could not find the config file: /home/vice/.config/composer/composer.json
To initialize a project, please create a composer.json file as described in the https://getcomposer.org/ "Getting Started" section
Running 1.10.1 (2020-03-13 20:34:27) with PHP 7.3.15-3 on Linux / 5.4.0-kali4-amd64
Loading composer repositories with package information
Downloading https://repo.magento.com/packages.json
Warning from repo.magento.com: You haven't provided your Magento authentication keys. For instructions, visit https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html
    Authentication required (repo.magento.com):
      Username: 238cfcfe268f5a9259156907005c7de5
      Password: 
Using HTTP basic authentication with username "238cfcfe268f5a9259156907005c7de5"
Downloading https://repo.magento.com/packages.json
Do you want to store credentials for repo.magento.com in /home/vice/.config/composer/auth.json ? [Yn] y
composer install 加-vvv是为了把所有的过程显示出来，要不然过程太久看不到详细的过程总会以为卡死了

三 (2）：将站点根目录下的auth.json.sample复制为auth.json，再编辑auth.json加入magento的Public Key/Private Key（此方案未验证）

本以为这样处理就正常了，结果还是回到刚开始的问题最初错误提示，再细看错误提示，（好像是用php7.3的版本，magento库需要的是php7.1-7.2的版本），composer install使用php的版本不对？本机安装了5.6、7.1、7.2、7.3四个不同的版本！那么用对php版本来执行是否可行？又查了老久，确认了三种处理方案，如下：

四 (1)：修改composer.json，在confing增加配置，指定php版本，如下

    "config": {
	 "platform" :{
	             "php": "7.1"
	 },
        "preferred-install": "dist",
        "sort-packages": true
    },
新增加platform这3行，再次执行还是用php7.3版本来执行composer install，结果失败

四 (2)：命令行指定php版本来执行composer install，如下

vice@srv /v/w/h/demo_m2_234 (master) [SIGINT]> 
php7.1 composer install -vvv
Could not open input file: composer
vice@srv /v/w/h/demo_m2_234 (master) [1]> 
/usr/bin/php7.1 composer install -vvv
Could not open input file: composer
vice@srv /v/w/h/demo_m2_234 (master) [SIGINT]> php7.1 /usr/bin/composer install -vvv
每三次，指定composer的脚本路径就能正常引用php7.1来调用composer install

php7.1 composer install 安装过程很花时间，等了好久，报了以下错误：

  - Installing zendframework/zend-http (2.11.2): Downloading https://api.github.com/repos/zendframework/zend-http/zipball/e15e0ce45a2a4f642cd0b7b4f4d4d0366b729a1a
Downloading (failed)       
    Download failed, retrying...
Downloading https://api.github.com/repos/zendframework/zend-http/zipball/e15e0ce45a2a4f642cd0b7b4f4d4d0366b729a1a
Downloading (failed)       
    Download failed, retrying...
Downloading https://api.github.com/repos/zendframework/zend-http/zipball/e15e0ce45a2a4f642cd0b7b4f4d4d0366b729a1a
Downloading (failed)       
Failed: [Composer\Downloader\TransportException] 0: The "https://api.github.com/repos/zendframework/zend-http/zipball/e15e0ce45a2a4f642cd0b7b4f4d4d0366b729a1a" file could not be downloaded: SSL: Connection reset by peer
Failed to enable crypto
failed to open stream: operation failed
    Failed to download zendframework/zend-http from dist: The "https://api.github.com/repos/zendframework/zend-http/zipball/e15e0ce45a2a4f642cd0b7b4f4d4d0366b729a1a" file could not be downloaded: SSL: Connection reset by peer
Failed to enable crypto
failed to open stream: operation failed
    Now trying to download from source
  - Installing zendframework/zend-http (2.11.2): Executing command (CWD): git --version

    Cloning to cache at '/home/vice/.cache/composer/vcs/https---github.com-zendframework-zend-http.git/'
Executing command (CWD): git clone --mirror 'https://github.com/zendframework/zend-http.git' '/home/vice/.cache/composer/vcs/https---github.com-zendframework-zend-http.git/'
Cloning e15e0ce45a2a4f642cd0b7b4f4d4d0366b729a1a
Executing command (CWD): git clone --no-checkout 'https://github.com/zendframework/zend-http.git' '/var/www/html/demo_m2_234/vendor/zendframework/zend-http' && cd '/var/www/html/demo_m2_234/vendor/zendframework/zend-http' && git remote add composer 'https://github.com/zendframework/zend-http.git' && git fetch composer && git remote set-url origin 'https://github.com/zendframework/zend-http.git' && git remote set-url composer 'https://github.com/zendframework/zend-http.git'
Failed: [Symfony\Component\Process\Exception\ProcessTimedOutException] The process "git clone --no-checkout 'https://github.com/zendframework/zend-http.git' '/var/www/html/demo_m2_234/vendor/zendframework/zend-http' && cd '/var/www/html/demo_m2_234/vendor/zendframework/zend-http' && git remote add composer 'https://github.com/zendframework/zend-http.git' && git fetch composer && git remote set-url origin 'https://github.com/zendframework/zend-http.git' && git remote set-url composer 'https://github.com/zendframework/zend-http.git'" exceeded the timeout of 300 seconds.

In Process.php line 1253:
                                                                               
  [Symfony\Component\Process\Exception\ProcessTimedOutException]               
  The process "git clone --no-checkout 'https://github.com/zendframework/zend  
  -http.git' '/var/www/html/demo_m2_234/vendor/zendframework/zend-http' && cd  
   '/var/www/html/demo_m2_234/vendor/zendframework/zend-http' && git remote a  
  dd composer 'https://github.com/zendframework/zend-http.git' && git fetch c  
  omposer && git remote set-url origin 'https://github.com/zendframework/zend  
  -http.git' && git remote set-url composer 'https://github.com/zendframework  
  /zend-http.git'" exceeded the timeout of 300 seconds.                        
                                                                               

Exception trace:
  at /usr/share/php/Symfony/Component/Process/Process.php:1253
 Symfony\Component\Process\Process->checkTimeout() at /usr/share/php/Symfony/Component/Process/Process.php:417
****省略******
/usr/share/php/Composer/Console/Application.php:113
 Composer\Console\Application->run() at /usr/bin/composer:62

install [--prefer-source] [--prefer-dist] [--dry-run] [--dev] [--no-dev] [--no-custom-installers] [--no-autoloader] [--no-scripts] [--no-progress] [--no-suggest] [-v|vv|vvv|--verbose] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--ignore-platform-reqs] [--] [<packages>...]

尝试以前的解决方案换composer的镜像，再执行composer install

vice@srv /v/w/h/demo_m2_234 (master)>composer config -g repo.packagist composer https://packagist.phpcomposer.com
vice@srv /v/w/h/demo_m2_234 (master)>php7.1 /usr/bin/composer install -vvv

四 (3) : 直接忽略php版本执行composer install --ignore-platform-reqs 或者 composer update --ignore-platform-reqs(此方案未验证)

最后感谢以下的分享：

Warning from repo.magento.com: Your Magento authentication keys are invalid. Please double-check your keys in your Marketplace account. For instructions, 错误的参考：

https://magento.stackexchange.com/questions/90983/how-to-use-the-new-repo-magento-com

https://community.magento.com/t5/Magento-2-x-Version-Upgrades/Invalid-credentials-for-https-repo-magento-com-packages-json/td-p/22287

Your requirements could not be resolved to an installable set of packages.与To enable extensions, verify that they are enabled in your，错误的参考

解决composer install遇到：Your requirements could not be resolved to an installable set of packages_JavaScript_IT-Andy-CSDN博客
首先看报错信息：如下图解决办法：直接忽略版本就是了composerinstall--ignore-pJavaScript
多个php版本的composer使用 - 周伯通的麦田 - 博客园
由于系统环境变量之前同事安装的laravel是5.1...php默认的环境变量是： 不想破话原有环境变量，因为现在新的项目是laravel5.4.。。所以在用默认composer require安装时
博客园