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<?php
 include("includes/db_config.php");
		//var_dump($_POST);exit();

if(!empty($_POST["cat_id"])){ 	
				$cat_id=$_POST["cat_id"];	
				$res = mysqli_query($conn,"select * from sub_categorydp where category_id='$cat_id' order by id asc");
				
				echo "<select class='simple-select2 form-control' name='div_name' id='div_idd'>";
				if(mysqli_num_rows($res) > 0 ){
					echo "<option value=".">"; echo "Select Sub Category";echo"</option>";
					while($row=mysqli_fetch_array($res)){
						
						echo "<option value=".$row["id"].">"; echo $row["sub_name"];echo"</option>";
					}
				echo "</select>";
				
				}else{
						echo "<option value=".">"; echo "Sub Category Not Available!";echo"</option>";
						echo "</select>";
						
				}
				
	} elseif(!empty($_POST["scat_id"])){ 
    
     $res1 = mysqli_query($conn,"select * from sub_sub_category where sub_category_id = ".$_POST['scat_id']." ORDER BY sub_sub_cat_name ASC");
    // Generate HTML of city options list 
    if(mysqli_num_rows($res1) > 0 ){
        echo '<option value="">Select Sub Sub Category</option>'; 
        while($row = mysqli_fetch_array($res1)){  
            echo '<option value="'.$row['id'].'">'.$row['sub_sub_cat_name'].'</option>'; 
        } 
    }else{ 
        echo '<option value=""> Sub Sub Category not available</option>'; 
    } 
}

Editing File: ajax_get_subsub_cat.php