As per yourv answer  below is
combo provides onCheck event. It occurs when option checkbox is clicked:
combo.attachEvent(“onCheck”,function(value,state){
    /your code here/
    return true
})
where i have add the function in my php file or dhtmlxcombo_extra.js file or sampl.html  file
plaese reply me so i will add this code  in my page
>> here i have add the function in my php file or dhtmlxcombo_extra.js file or sampl.html file
The code should be added on the page where combo is initialized after the initialization. 
combo = new dhtmlXCombo(…);
combo.attachEvent(“onCheck”,function(value,state){ 
/your code here/ 
return true 
})