PHP: unable to locate functions being used (seemingly not defined)

I'm looking at someone else's code. There's class that calls functions like GetSku(), WriteOrder($order), etc., but I unable to find where these are located. I've checked for …

• Other script or class inclusions: no explicit or magic functions (__call) defined
• grep -rHn "n *GetSku" /path/to/codebase: returns nothing
• Sublime's cTags: nothing found
• Original developer's knowledge base: he returned an error when I made an email request

These mysterious function are called literally like…

$sku = GetSku();
WriteOrder($order);

What could possibly define and them them work? Custom magic functions somehow? Some kind of dynamic functions? I'm scratching my head like a monkey with an iPhone.

Maybe the class was included in a php file that is including some external libraries? The functions might be taken from these libraries.