jQuery Grid Plugin - jqGrid

I want to use custom function to check some field like this.

/-------------------------------------------------------------------------------------------------------/

{name:'username',index:'username', width:80,align:"laft",
                editable:true,
                formoptions:{rowpos:3},                
                editrules: {edithidden: true, required: true,custom:true,custom_func:usercheck},
                searchoptions: { sopt: ['bw']}
 },

function usercheck(value,username){
        $.ajax({
           type: "POST",
           url: "<?=ROOT_PATH?>/users/usercheck.php",
           data: {username:value},
           success: function(msg){
              return [false,msg];
           }
        });
    }

/-------------------------------------------------------------------------------------------------------/

usercheck.php

// db process

and than

if($soemcheck){

    echo '  ""  '

}else{

   echo '  "This id already taken.."  '

}

/-------------------------------------------------------------------------------------------------------/

When I do this. some message show me like this.

" Custom function should return array! "

If I do not use ajax, It working to check some field. but ajax is not working.

Tony, Please help me..

Thanks a lot...