including customer functions

I have looked everywhere but the answers are jibberdish for me. So here’s my problem:

For my site I have a range of custom made functions.
They are now at the top of the functions.php from the theme I use.
That means that every time I upgrade the theme I have to add them there again.
Is there a way that I can put these functions in a separate cust_functions.PHP file and include that file with one line in the functions.php?
One line is easier to add and maintain than a whole range of functions…

You could add these as a must-use plugin. /wp-content/mu-plugins/

See: Knowledge Base » Hacking s2Member® Via Hooks/Filters

Right… (sigh) That is as clear as muddy water.
This question had nothing to do with s2 other than that there are quite a few good php coders out here.
I was more thinking about adding one line to my theme functions.php to make the functions I created available. I guess now that there is not such a thing so I will have to keep merging my own functions into the functions.php. (sigh, talking about efficiency)

In VFP there was a command like: “Set procedure to ABC additive” adding all kind of function containing programs into one file… again… those were the good old productive days… Now I have to deal with php…

Try adding your functions to a PHP file named whatever you want, and put the file in the /wp-content/mu-plugins/ directory, they’ll be loaded by WordPress without you doing anything else. http://codex.wordpress.org/Must_Use_Plugins