How insert if/elseif/else statement into callback

Hi, how can I insert if/elseif/else into callback? This is my idea:

uicontrol('Style','checkbox',...
            'Units','normalized',...
            'Position',[0.4 0.05 0.3 0.1],...
            'String','Two!',...
            'Callback',
            if get(handles.checkbox,'Value')==1
               a=1
            if get(handles.checkbox,'Value')==0
               a=0;
            end; );

Count of checkboxes is always different. Thank you