PHP warnings / errors show for various reasons. In case you get such PHP messages on your website, you can do the following:
1) If it is just a warning and your scripts work properly you can disable the php warnings. This can be done with a custom php.ini file containing the following line:
display_errors = off
You can create this file using your cPanel > File Manager.
Please note that you should place this file in every folder where you need the custom settings to apply.
2) Try to resolve the issue. PHP warning / error messages are quite descriptive and you can often find the solution for the issue in the message. You can also Google-search for possible solutions of your problem. Almost certainly someone before you has had the same problem and found a solution for it.