How to run composer from anywhere?

composer.phar can be ran on its own, no need to prefix it with php. This should solve your problem (being in the difference of bash's $PATH and php's include_path).

You can do a global installation (archived guide):

Since Composer works with the current working directory it is possible to install it in a system-wide way.

1. Change into a directory in your path like cd /usr/local/bin
2. Get Composer curl -sS https://getcomposer.org/installer | php
3. Make the phar executable chmod a+x composer.phar
4. Change into a project directory cd /path/to/my/project
5. Use Composer as you normally would composer.phar install
6. Optionally you can rename the composer.phar to composer to make it easier

Update: Sometimes you can't or don't want to download at /usr/local/bin (some have experienced user permissions issues or restricted access), in this case, you can try this

1. Open terminal
2. curl -sS http://getcomposer.org/installer | php -- --filename=composer
3. chmod a+x composer
4. sudo mv composer /usr/local/bin/composer

Update 2: For Windows 10 and PHP 7 I recommend this tutorial (archived). Personally, I installed Visual C++ Redistributable for Visual Studio 2017 x64 before PHP 7.3 VC15 x64 Non Thread Safe version (check which versions of both in the PHP for Windows page, side menu). Read carefully and maybe the enable-extensions section could differ (extension=curl instead of extension=php_curl.dll). Works like a charm, good luck!