Could not open input file: composer.phar
php composer.phar update
However, if you install it globally, you can make composer itself executable, so you can call it without php (and don't have to download it for every project). In other words, you can use composer like this:
If you don't, either download the composer.phar to your project directory, or install Composer globally (I'd recommend the latter).
If you have Composer installed globally, simply run composer update instead of php composer.phar update.
Installing Composer locally simply means that you are downloading a file (composer.phar - which is a PHP Archive) into your project directory. You will have to download it for every project that requires Composer.
It is helpful to know that there are two ways to install (and use) Composer: locally as a file in your project directory, or globally as a system wide executable.
Like a regular PHP file that you want to execute on the command line, you will have to run it with PHP:
Which basically tells the php executable to run the file composer.phar with update as argument.