How to use the value of input fields in an if statement - php

Question

I have two input fields. One is to search auto suggest product names using jquery.autocomplete, and in my database I have column name product left, and I want to display it in the second input, is that possible?
For example: I have a product called apple with product left of 30. apple will display in the first input field after typing, and at same time 30 must appear in the second field.
<form action="sales.php" method="post">
<input name="productlist" type="text" id="productlist" size="20"/>
<input name="productleft" type="text" value=""/>
</form>
call jquery ui
<script type="text/javascript" src="js/jquery.autocomplete.js"></script>
<script>
$(document).ready(function(){
$("#productlist").autocomplete("psuggest.php", {
selectFirst: true
});
});
</script>
inside psuggest.php
<?php
$q=$_GET['q'];
$my_data=mysql_real_escape_string($q);
$mysqli=mysqli_connect('localhost','root','','saganatracker') or die("Database Error");
$sql="SELECT pdesc FROM products WHERE pdesc LIKE '%$my_data%' ORDER BY pdesc";
$result = mysqli_query($mysqli,$sql) or die(mysqli_error());
if($result)
{
while($row=mysqli_fetch_array($result))
{
echo $row['pdesc']."\n";
}
}
?>

Answer 1

Use parameter onselect from .autocomplete() to bring the number of item left :
$(document).ready(function(){
$("#productlist").autocomplete("psuggest.php", {
selectFirst : true,
onselect : function(value, data) {
$.post("myPhPfileWhichBringNumberOfItem.php", { product : value }, function(count) {
$("#productleft").val(count); // Don't forget to mention an id for the input first
}
}
});
});
Use JQuery post at the moment a value is selected, and then use a php script which open your database, go to the table and select number of product left (not sure if you should use value or data, please someone correct if I am wrong).