I ran the update command on Composer and received the following error:

[Composer\Downloader\TransportException]
The "http://packagist.org/p/symfony/class-loader$78539d95ddac0cc02514ed9cdf1515023dcd289b4f7e03ed948e868a1b02acf2.json" file could not be downloaded: failed to open stream: HTTP request failed!


Seems odd considering I was able to install Composer using php cli. I believe my Apache configuration, which forces all traffic over https, is to blame. I found the following solution in the GitHub forums.

Modify your composer.json file to include the following:

"repositories": [
    {
        "packagist": false
    },
    {
        "type": "composer",
        "url": "https://packagist.org/"
    }
],

Then run the update command.  This tells Composer the repository is at https://packagist.org, instead of the default http variation.

Credit: https://github.com/composer/composer/issues/1992 (specifically https://github.com/cs278)