Composer is a PHP dependency manager. To install it globally on CentOS follow the steps below.

First, go to the /tmp directory

cd /tmp  

Download the composer.phar file

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

Move it to /usr/local/bin/

mv composer.phar /usr/local/bin/composer  

now you should be able to use the command composer globally.