I just downloaded Netbeans for Ubuntu. Now I only have Java, how can I add PHP to it?
Go to Tools->Plugins
On the Installed tab check
PHP and click on Activate. Restart NetBeans.
Note: I assume you have downloaded the appropriate package containing PHP tools.
On 7.0.1 go to Tools->Plugins
click the Settings tab and Activate the Update Centers.
Now click the Available Plugins tab and you should see PHP
Install and Reboot Netbeans.
On 8.1, same as above for 7.0.1 but some more precision : on the Settings tab you need to tick the NetBeans Distribution checkbox to see PHP in the Available Plugins list.