I've got a problem with a new WordPress install where some functionlity is not working and the plugin developer has asked me to see if the php curl and mbstring extensions are enabled on the server.
HOw would I go about doing that and possibly installing them if they are not there?
The way to install these PHP extensions depends on which OS you're using.
For Ubuntu 14.04 (you only need to install the curl extension as mbstring should be built in):
sudo apt-get install php5-curl
For Ubuntu 16.04:
sudo apt-get install php7.0-curl
sudo apt-get install php7.0-mbstring
For Centos 7 (you only need to install the mbstring extension as curl should be built in):
yum install php-mbstring
Alternatively, to avoid problems related to missing PHP extensions, you could manage your server with ServerPilot which installs all common PHP extensions. The full list is here:
https://serverpilot.io/community/articles/available-php-extensions.html