Here is an example with a solution provided for you:
I have made some necessary adjustments to your HTML, including the addition of "ids":
//HTML file
<tr>
<td><label for="item1">Item 1</label></td>
<td><label for="lblitem1price">$25.00</label></td>
<td><input type="text" id="item_1_qty" name="txtitem1qty" value="0" maxlength="10" size="3"></td>
<td><input type="text" id="item_1_tax" name="txtitem1tax" maxlength="10" size="3" readonly></td>
<td><input type="text" id="item_1_totalprice" name="txtitem1totalprice" maxlength="10" size="3" readonly></td>
</tr>
Javascript within the HTML file:
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.0.0-beta1/jquery.min.js"></script>
<script>
function calculate(el){
var quantity = el.val();
var id = el.attr("id").replace("item_","").replace("_qty","");
var data = {quantity: quantity,id:id};
var targetTax = $("#item_"+id+"_tax");
var targetTotalPrice = $("#item_"+id+"_totalprice");
$.post("sandbox.php",data,function(response){
response = JSON.parse(response);
targetTax.val(response.tax);
targetTotalPrice.val(response.totalprice);
});
}
var qty = $("#item_1_qty");
qty.on("keyup",function(){
calculate($(this));
});
</script>
The filename in my code is called sandbox, but you can rename it to calculate.php;
//sandbox.php
<?php
//It's advisable that the data resides in a database
$items = array(
array("id"=> 1, "label"=> "Item1","value"=>25.0)
);
$tax = 0.3;
$id = isset($_POST['id'])?$_POST['id']:0;
if(!is_numeric($id)){
$id = 0;
}
$quantity = isset($_POST['quantity'])?$_POST['quantity']:1;
if(!is_numeric($quantity)){
$quantity = 1;
}
$finalValue = 0;
$currentItem = null;
foreach($items as $item){
if($item["id"] == $id){
$currentItem = $item;
}
}
if($currentItem != null){
$finalValue = $quantity * $currentItem["value"];
$finalValue = ($finalValue * $tax) + $finalValue;
}
$return = array(
'quantity' => $quantity,
'tax' => $tax,
'totalprice' => $finalValue
);
print json_encode($return);
UPDATE:
The process of finding a product in the database serves as just an example that you can implement:
$db = new mysqli('localhost', 'user', 'pass', 'demo');
if($db->connect_errno > 0){
die('Unable to connect to database [' . $db->connect_error . ']');
}
$sql = "SELECT p.id,p.value FROM Products p WHERE p.id = ?";
$stmt = $mysqli->prepare($sql);
$stmt->bind_param('i', $id);
$stmt->execute();
$stmt->close();
if(!$result = $stmt->get_result()){
die('There was an error running the query [' . $db->error . ']');
}
$currentItem = $result->fetch_array();
$tax = 0.3;
$id = isset($_POST['id'])?$_POST['id']:0;
if(!is_numeric($id)){
$id = 0;
}
$quantity = isset($_POST['quantity'])?$_POST['quantity']:1;
if(!is_numeric($quantity)){
$quantity = 1;
}
$finalValue = 0;
if($currentItem != null){
$finalValue = $quantity * $currentItem["value"];
$finalValue = ($finalValue * $tax) + $finalValue;
}
$return = array(
'quantity' => $quantity,
'tax' => $tax,
'totalprice' => $finalValue
);
print json_encode($return);