[Omega-300]

The 'GetPlayerPos' is not an callback but rather a usual function, the references are gained by Amx_GetAddr.
Though I suggest you to be familiar with C++ quite a bit and know how AMX works so you don't just do it with trial-and-error approach(which is not good at all) and actually understand why something is done!
Example:

Код:

cell *pFirstParam = NULL;
if(amx_GetAddr(amx, params[1], &pFirstParam) == AMX_ERR_NONE)
{ 
    *pFirstParam = 123;
}