Installing Composer on OS X

Composer is a cross-platform dependency manager for PHP libraries. This article will explain how to install it on OS X and add an alias so you can use it from anywhere. 

Installing

The first step is to download Composer, which will effectively create a Phar (PHP Archive) file called composer.phar. From your terminal, run the following command: 

curl -sS https://getcomposer.org/installer | php

Installation Errors

In some cases, you might receive the following error: 

Some settings on your machine make Composer unable to work properly.
Make sure that you fix the issues listed below and run this script again:

The detect_unicode setting must be disabled.
Add the following to the end of your `php.ini`:
detect_unicode = Off

A php.ini file does not exist. You will have to create one.
If you can not modify the ini file, you can also run `php -d option=value` to modify ini values on the fly. You can use -d multiple times.

For OS X 10.8, the php.ini file should be located in /private/etc/. If you don’t have one already, you can copy the default one: 

sudo cp /private/etc/php.ini.default php.ini

Then, edit the file and append the following to it: 

detect_unicode = Off

Save the file and rerun the curl command above. 

Running Composer

The resulting file will be called composer.phar, a PHP Archive that can be executed directly via PHP. However, in my case, I want Composer to be accessible globally by simply typing composer. To do this, move it to /usr/bin/ and create an alias: 

sudo mv composer.phar /usr/bin/
vim ~/.bash_profile

Add this to your .bash_profile. It may be empty or non-existent, so go ahead and create it: 

alias composer="php /usr/bin/composer.phar"

Now, relaunch your terminal and you’ll be able to access Composer simply by calling composer.

reference : http://www.abeautifulsite.net/installing-composer-on-os-x/