This tutorial will take you through the steps on how to check what PHP extensions are installed on your server to meet PHP extension requirements for Magento 2.0.
First you will need to login to putty; it does not matter if you login as a root user or not. After you have logged in type “php -m” and hit the enter key. A list of PHP modules (extensions) that are installed on your server will appear. Compare the list to Magento’s 2.0 Required PHP Extensions list. In my case scenario we are missing the extension mhash but come to find out that mhash is obsoleted by hash. So hash has replaced mhash which means we have all of the required PHP extensions for Magento 2.0.
Required PHP extensions:
bc-math (enterprise edition only)
gd, ImageMagick 6.3.7 (or later) or both
mhash (obsolete see hash)
opcache (not required; highly recommended)
json (PHP 7 only)
iconv (PHP 7 only)