PHP: MySQL values call based on dropdown - php

Question

So what Ia m trying to do is to call a value from my server and input it in a variable based on what I have chosen from the dropdown box and then I want to take those variables and calculate a new output through javascript (I don't think that I want help for the last part but I like being up-front). Anyway, here is the code
$db_name = 'prod_chrotex_db';
$conn = new mysqli($host, $user, $pass, $db_name);
$query = mysqli_query($conn, "SELECT * FROM table1 INNER JOIN table2 ON id ");
echo '<select name="TITLE">';
while ($row = mysqli_fetch_array($query)) {
echo '<option value="' . $row['title'] . '">' . $row['title'] . '</option>';
$rows = mysqli_fetch_assoc($query);
echo 'ID: ' . $rows['ID'] ;
$value = $rows['values']
};
echo "</select>";

Answer 1

$db_name = 'prod_chrotex_db';
$conn = new mysqli($host, $user, $pass, $db_name);
$query = mysqli_query($conn, "SELECT * FROM table1 INNER JOIN table2 ON id ");
echo '<select name="TITLE" id="selectb" onchange="callfunction();>';
while ($row = mysqli_fetch_array($query)) {
echo '<option value="' . $rows['ID'] . '" ">' . $row['title'] . '</option>';
};
echo "</select>";
?>
<script>
function callfunction(){
var val = document.getElementById('selectb').value();
// You get the selected value in val variable. Here you can calculate what you want
}
</script>