Download Composer to Mac OSX

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('SHA384', 'composer-setup.php') === '544e09ee996cdf60ece3804abc52599c22b1f40f4323403c44d44fdfdd586475ca9813a858088ffbc1f233e9b180f061') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"

Downloading the Composer Executable#
Composer offers a convenient installer that you can execute directly from the commandline. Feel free to download this file or review it on GitHub if you wish to know more about the inner workings of the installer. The source is plain PHP.

There are in short, two ways to install Composer. Locally as part of your project, or globally as a system wide executable.

Locally#
To install Composer locally, run the installer in your project directory. See the Download page for instructions.

The installer will check a few PHP settings and then download composer.phar to your working directory. This file is the Composer binary. It is a PHAR (PHP archive), which is an archive format for PHP which can be run on the command line, amongst other things.

Now run php composer.phar in order to run Composer.

You can install Composer to a specific directory by using the --install-dir option and additionally (re)name it as well using the --filename option. When running the installer when following the Download page instructions add the following parameters:

php composer-setup.php --install-dir=bin --filename=composer
Now run php bin/composer in order to run Composer.

Globally#
You can place the Composer PHAR anywhere you wish. If you put it in a directory that is part of your PATH, you can access it globally. On unixy systems you can even make it executable and invoke it without directly using the php interpreter.

After running the installer following the Download page instructions you can run this to move composer.phar to a directory that is in your path:

mv composer.phar /usr/local/bin/composer
If you like to install it only for your user and avoid requiring root permissions, you can use ~/.local/bin instead which is available by default on some Linux distributions.

Note: If the above fails due to permissions, you may need to run it again with sudo.

Note: On some versions of OSX the /usr directory does not exist by default. If you receive the error "/usr/local/bin/composer: No such file or directory" then you must create the directory manually before proceeding: mkdir -p /usr/local/bin.

Note: For information on changing your PATH, please read the Wikipedia article and/or use Google.

Now run composer in order to run Composer instead of php composer.phar.