PHP：无法找到正在使用的函数（看似未定义）

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.

Could you check on the actual server if this attribute is set in php.ini?

auto_prepend_file 

Specifies the name of a file that is automatically parsed before the main file. The file is included as if it was called with the require function, so include_path is used.

The special value none disables auto-prepending.

This could be the method how non-existing functions could be loaded from external files outside the project scope.