如何使用数组更新列

I have a table whose primary key is a column named St_ID. I want to update another column in that same (ID) using values stored in an array. But when I try the code below, the result is a new record with an St_ID value of '0' and all other columns are empty.

Note, courseID is a value chosen through a drop down list. Do you have any idea where I went wrong?

for ($i = 0; $i < $count; $i++){
   $Student = $foo[$i];
   $res = mysql_query("SELECT St_ID FROM student WHERE St_ID='$Student' ");
   while($row = mysql_fetch_array($res))
   {
      $sql = "INSERT INTO student (ID) VALUES
      ('" . $_POST[$row['courseID']] . "')";
   }
}
if (!mysql_query($sql,$connectdb))
{
   die ('Error :'.mysql_error());
}
echo "The Students are add to the course <br />";

Here simplified code, with only one query

$where = "'".implode("','", $foo)."'";
$res = mysql_query("UPDATE student set ID = courseID WHERE St_ID IN ($where)") 
         or die('Error :'.mysql_error());

 echo "The Students are add to the course <br />";

you select St_ID but try to insert courseID

in this line

$sql = "INSERT INTO student (ID) VALUES
     ('" . $_POST[$row['courseID']] . "')";

SELECT St_ID FROM student WHERE St_ID='$Student' 
INSERT INTO student (ID) VALUES ...

If you want to update that record you chose, you must use the UPDATE sql command instead;

UPDATE student 
SET ID=...
WHERE St_ID='$Student'