function_exists() - find the file containing a given function in an
string function_exists( string str, object ob );
Return the file name of the object that defines the function 'str' in
object 'ob'. The returned value can be other than 'file_name(ob)' if
the function is defined by an inherited object.
0 is returned if the function was not defined.
Note that function_exists() does not check shadows.
If no object is passed as the second argument, this efun will default
call_other(3), call_out(3), functionp(3), valid_shadow(4)