The most recent Joomla! version – Joomla! 3.4 – does not support PHP 7. Support for PHP 7 will be added in Joomla! 3.5 in early 2016. Joomla! users will be able to test the improvements in PHP 7 with their websites in February 2016, if all goes as planned.
More info: The introduction of type hints and the now-reserved words “int”, “string” and “float” is currently causing problems for certain CMSs. For instance, version 3.4 of Joomla! is not compatible with PHP 7 because the type designation “string” is used in a manner that is not permitted in PHP 7. The Joomla! community has since corrected this usage and will publish a compatible version, Joomla! 3.5, at the beginning of 2016.