c++ convert from LPCTSTR to const char *

Since you're using MFC, you can easily let CString do an automatic conversion from char to TCHAR:

MyFunction(CString(wChar));

This works whether your original string is char or wchar_t based.

Edit: It seems my original answer was opposite of what you asked for. Easily fixed:

MyFunction(CStringA(wChar));

CStringA is a version of CString that specifically contains char characters, not TCHAR. There's also a CStringW which holds wchar_t.

LPCTSTR is a pointer to const TCHAR and TCHAR is WCHAR and WCHAR is most probably wchar_t. Make your function take const wchar_t* if you can, or manually create a const char* buffer, copy the contents, and pass that.

When UNICODE is defined for an MSVC project LPCTSTR is defined as const wchar_t *; simply changing the function signature will not work because whatever code within the function is using the input parameter expects a const char *.

I'd suggest you leave the function signature alone; instead call a conversion function such as WideCharToMultiByte to convert the string before calling your function. If your function is called several times and it is too tedious to add the conversion before every call, create an overload MyFunction(const wchar_t *wChar). This one can then perform the conversion and call the original version with the result.