window.onload=function(){
       if(val != 3) {
        document.getElementById("startdatebox").disabled = true;
        document.getElementById("enddatebox").disabled = true;
    } else {
        document.getElementById("startdatebox").disabled = false;
        document.getElementById("enddatebox").disabled = false;
    }

    } ​

   function myFunc(val) {
    if(val != 3) {
        document.getElementById("startdatebox").disabled = true;
        document.getElementById("enddatebox").disabled = true;
    } else {
        document.getElementById("startdatebox").disabled = false;
        document.getElementById("enddatebox").disabled = false;
    }

 //  alert( document.getElementById("dateRange").value);

}

...
       echo $this->Form->input('startDate',array('id'=>'startdatebox','label' => 'Start Date','class'=>'datepicker', 'type'=>'text','style'=>'width:100px;height:30px','value' => $datestart));

 echo $this->Form->input('endDate',array('id'=>'enddatebox','label' => 'End Date','class'=>'datepicker', 'type'=>'text','style'=>'width:100px;height:30px','value' => $dateend));




    $selectoption=array(1=>'Today',0=>'Fortnight',2=>'Monthly',3=>'Custom Range');
       echo $this -> Form -> input('dateRange',
               array('id'=>'dateRange2','label' => '<h6>Date Range</h6>','type' => 'radio',
              'value' =>$dateSelect,'options' =>  $selectoption,  'onclick'=> 'myFunc(this.value)'));