Where i can create a custom function which will available everywhere

[Saroha]

Hi Everyone,
               I want to create a function that will be available on every page in WHMCS. Please tell me where I can create that function because if I create a custom function and include it in my custom pages then it's not available on WHMCS default pages. I can use hook because I want to use it on every page with output on the .tpl output in the middle of the page at my specific section. 
For example. I want to add some extra details in my menu which is available on my custom pages as well as default pages.
but I want to do it without enabling/using the PHP tag in the template file.

Edited February 12 by saroha