Have you encountered any of these composer errors?

file_put_contents(/home/vagrant/.composer/cache/repo/https---packagist.laravel-china.org/packages.json): failed to open stream: Permission denied
https://packagist.laravel-china.org could not be fully loaded, package information was loaded from the local cache and may be out of date

Failed to decode response: zlib_decode(): data error
Retrying with degraded mode, check https://getcomposer.org/doc/articles/troubleshooting.md#degraded-mode for more info

- Installing phpunit/phpunit (6.5.8): Downloading (failed)
Downloading (failed)
Downloading (failed)    Failed to download phpunit/phpunit from dist: The "https://files.phpcomposer.com/files/sebastianbergmann/phpunit/4f21a3c6b97c42952fd5c2837bb354ec0199b97b.zip" file could not be downloaded: SSL operation failed with code 1. OpenSSL Error messages:
error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed
Failed to enable crypto
failed to open stream: operation failed
    Now trying to download from source

In my case, it was the CA Cert file that was the culprit
Open terminal and run php -r "var_dump(openssl_get_cert_locations());"
Note the location of default_cert_file. It might say \usr\local\ssl\cert.pem
Verify that that file exists. If not, download the cacert.pem file from https://curl.haxx.se/ca/cacert.pem and place it in somewhere preferably in /usr/local/ssl
You now have a file is usr/local/ssl/cacert.pem
Open your php.ini and add the location of the file openssl.cafile=cacert.pem
Run composer config --global cafile "/usr/local/ssl/cacert.pem"
If it's of any use to others, I use valet+ 1.0.26, composer 1.8.4 OSX Movajes 10.14.3, php7.3 and simply specifying the cert composer looks for with:
composer config --global cafile "/usr/local/etc/openssl/cert.pem"

composer config --global cafile "C:/AMP_ENV/php-7.1.15/ca/cacert.pem"
composer config --global --list

Installation failed, reverting ./composer.json to its original content.
The following exception is caused by a lack of memory or swap, or not having swap configured
Check https://getcomposer.org/doc/articles/troubleshooting.md#proc-open-fork-failed-errors for details
PHP Warning:  proc_open(): fork failed - Cannot allocate memory in phar:///usr/local/bin/composer/vendor/symfony/console/Application.php on line 952
Warning: proc_open(): fork failed - Cannot allocate memory in phar:///usr/local/bin/composer/vendor/symfony/console/Application.php on line 952
  [ErrorException]
  proc_open(): fork failed - Cannot allocate memory

sudo dd if=/dev/zero of=/var/swap.1 bs=1M count=1024
sudo mkswap /var/swap.1
sudo swapon /var/swap.1

root@test2:/work/usrc$ composer require qcloud/cos-sdk-v5
./composer.json has been updated
The "extra.asset-installer-paths" option is deprecated, use the "config.fxp-asset.installer-paths" option
Loading composer repositories with package information
Updating dependencies (including require-dev)
Killed

free -m
sudo mkdir -p /var/_swap_
cd /var/_swap_
sudo dd if=/dev/zero of=swapfile bs=1M count=2000
sudo mkswap swapfile
sudo swapon swapfile
sudo echo "/var/_swap_/swapfile none swap sw 0 0" >> /etc/fstab
free -m