How to create dynamic Chained dropdown list in codeigniter

1. If you want the content of your dropdowns to be dynamic then I believe you can't achieve this without accessing your database by specifying your queries.
2. From what I understood in your question, you want a dynamic dropdown in which its content is filtered by its preceding dropdown. Then your best bet would be to use AJAX, as PHP is only able to run server accessing functions without POST and GET which needs refreshing the page.

here's an ajax demo

<script type="text/javascript"> 




function getXMLHTTP() { //function to return the xml http object
var xmlhttp=false;  
try{
    xmlhttp=new XMLHttpRequest();
}
catch(e)    {       
    try{            
        xmlhttp= new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch(e){
        try{
        xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
        }
        catch(e1){
            xmlhttp=false;
        }
    }
}

return xmlhttp;
}



function getDoc(strURL) {       

var req = getXMLHTTP();

if (req) {

    req.onreadystatechange = function() {
        if (req.readyState == 4) {
            // only if "OK"
            if (req.status == 200) {                        
                document.getElementById('my_div').innerHTML=req.responseText;                       
            } else {
                alert("There was a problem while using XMLHTTP:\n" + req.statusText);
            }
        }               
    }           
    req.open("GET", strURL, true);
    req.send(null);
}

}
                                </script>

Here's where the magic happens. In my first dropdown I placed an onChange attribute. This calls my getDoc javascript function which has a PHP FUNCTION as parameter.

    <select name="docSpec" id="docSpec" onChange="getDoc('getSpec.php?spec='+this.value)">

            <option>pick a specialization</option>
            <option value="General">General</option>
            <option value="pediatrics">pediatrics</option>
            <option value="Physician">Physician</option>
            <option value="Cardiologist">Cardiologist</option>
            <option value="Pulmonary">Pulmonary</option>

    </select>
    <div id="my_div" style="display:inline;">       
                <!--content from getSpec.php goes here--->                              
    </div>

Now you need a DIV in your HTML which takes the data from another page then returns it in your current page as a DOM. This way your original page doesn't refresh but still gets data from the server.

getSpec.php to show what happens under the php sheets

require_once('dbcon.php');
$query="select doctorName from doctors where specialty= '$spec'";
$result=mysqli_query($conn,$query);

?>
<select name="docName">

<?php 

    while( $row = mysqli_fetch_row($result)) { 

echo "<option type='hidden' value='$row[0]'>$row[0]</option>";
} ?>

 </select> 

I hope this code helps. You can tweak this to your needs.