I had same issues but this what I did:
I copy paste this into the terminal php -r “copy(‘http://getcomposer.org/installer’, ‘composer-setup.php’);” on the terminal which download the composer to the directory where you are currently In.
(the problem was causing because of the https protocol)
And then php composer-setup.php –install-dir=/usr/local/bin –filename=composer run this command, which installs the Composer to my bin folder so that i could run it globally.
Then you have to unlink the composer php -r “unlink(‘composer-setup.php’);”
Finally check the see the Composer installed composer -V.
Good Luck !