如何在查询中使用的同一文件中提取id属性

In my website I have drop down box and I made event to know which element selected from the user .. and it works . I can retrieve the value and print it on the screen using id attribute !! This function will work when user Select from drop down

<script>
function SelServ(){
    var x = document.getElementById("services").value;
    document.getElementById("chosenServ").innerHTML =x;


}
</script>

The drop down box reads from database

echo "<select name = \"services\" id = \"services\" onchange = \"SelServ()\">";
//my connection
$myCon = new mysqli("localhost","root","","mss");
if(mysqli_connect_errno()){
    printf("Connect Failed : %s
",mysqli_connect_error());
}
else{
    $sql = "SELECT ServID ,ServName FROM service";
    $result =  mysqli_query($myCon,$sql);
    if($result){
        while($StaffArr = mysqli_fetch_array($result,MYSQLI_ASSOC)){

            $servName = $StaffArr['ServName'];
            $ServiceID = $StaffArr['ServID'];
            echo "<option value = \"".$ServiceID."\">".$servName."</option>";
        }
    }
    else{
        printf("Couldn't retrive records ! <br /> %s",mysqli_error($myCon));
    }
}
echo "</select>";

And I'm using div to retrieve selected item

$servIDHTML = "<div id = \"chosenServ\"></div>";

I want to extract the value of id and included in sql statement to affect in seconed drop down box in the same page !! Any idea?

You must use ajax:-

Let's say you have 2 dropdown parent_id and children:-

<select name="parent_id" id="parent_id">
  <option value="">Select ID</option>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
</select>

<select name="child" id="child">
  <option value="">Select Children</option>
</select>

Include the following ajax at the bottom of your page:-

<script>
 $('#parent_id').change(function(){

    var parentId = $('#parent_id').val();

    $.ajax({
        url: "ajax.php",
        type: "post",
        data: {parentId: parentId}, 
        success: function(data) {
            $('select#child').html(data);
        }
    });
});
</script>

PHP file where you can execute the SQL query based on your selection and return the result in options:-

ajax.php

if($_POST){
    $parentId = $_POST['parentId'];
    $sql = "SELECT * FROM TABLENAME WHERE parent_id=".$parentId;
    /*
     Fetch the result, prepare options for select and echo them
    */
}

</div>