在mysql中存储国家/地区下拉框中的国家/地区ID，但它在db中存储为0

//This is my php to view dropdown box  <?php
include('connect.php');
//for retriving data from DB and show it in drop down box
    $query="SELECT cname FROM country";
    $result = mysqli_query ($con, "$query");
    echo "<select name=country value=''>";
      while($r=mysqli_fetch_row($result))
      {
        echo "<option value='$r[0]'> $r[0] </option>";
      }
      echo "</select>";
?>

But when i am storing in DB my country_id is always 0.

You are not fetching country id from SQL Query

Change

$query="SELECT cname FROM country";

To

$query="SELECT cid, cname FROM country"; // Update cid with your country id field

Therefore $r[0] is getting blank value.

I am going with Pupil's answer to change the query to

$query="SELECT cid, cname FROM country"; // Update cid with your country id field

and then change this

  echo "<select name=country value=''>";

to

 echo "<select name=country>"; //remove value=''

EDIT:

Please check your column type may be it is "INT" when you try to store text it stores 0.

Use this code for your reference

//This is my php to view dropdown box 
 <?php
    include('connect.php');
    //for retriving data from DB and show it in drop down box
    $query="SELECT countryid,cname FROM country";
    $result = mysqli_query ($con, "$query");
    echo "<select name='country'>";
    while($r=mysqli_fetch_row($result))
    {
        echo "<option value='$r[countryid]'> $r[cname] </option>";
    }
    echo "</select>";
 ?>