COMPOSER_HOME not working

[khromov]

I tried setting COMPOSER_HOME to the suggested value:

define('COMPOSER_HOME', '/tmp/composer');

Still, it doesn't work:

Cannot create cache directory /var/www/.composer/cache/repo/https---packagist.org/, or directory is not writable. Proceeding without cache
Cannot create cache directory /var/www/.composer/cache/files/, or directory is not writable. Proceeding without cache

Composer version:

Composer version 1.2.0 2016-07-19 01:28:52

I don't know if this is handled with environmental variables but perhaps my nginx + php-fpm setup isn't able to set them properly?