在mac上下载composer时出现SSL错误

I'm bit of a newb in macOS and I'm struggling to install composer on my macOS system. Whenever I try to execute

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"

in my terminal from here, I'm getting this error.

I tried to ask google, but there was no success for me.

If you can help me with this I would be very grateful. Also sorry for my english, I'm not a native speaker.

Download the composer phar from directly https://getcomposer.org/download/1.2.1/composer.phar and now just run php composer.phar in order to run Composer. If you want it globally ,follow the link https://getcomposer.org/doc/00-intro.md#globally

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

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.

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