How to secure file uploads? (PHP setting?)

I have two WHM/cPanel VPS servers mainly hosting WordPress sites. On one of them (server 1), if you want to upload or update a plugin, you have to enter the cPanel account's FTP details in WP to enable the upload. On the other one (server 2), it just does it automatically without the need to enter the FTP login details. I'm pretty sure I set up server 1 to behave like that, and I want the same behaviour on server 2, but I can't for the life of me remember or find out how to do it. Can someone please point me in the right direction? I basically just want to find the setting where I can set PHP uploads to need an FTP login before you can upload. Thanks