Returning MySQL table values from PHP script to Javascript function - Live Graphing

Question

I have been searching similar problems for hours but with no avail.

I am using Highcharts to update a graph every 3 seconds with the last entry of a specific MySQL table. I am using the example Javascript code as a guide. Here is the snippet of code I am concerned with

var chart;
$('#container').highcharts({
  chart: {
    type: 'spline',
    animation: Highcharts.svg, // don't animate in old IE
    marginRight: 10,
    events: {
      load: function() {
        // set up the updating of the chart every 3 seconds
        var series = this.series[0];
        setInterval(function() {
          var x = (new Date()).getTime(), // current time - adjust to align with data vals
              y = getyval();
          series.addPoint([x, y], true, true);
        }, 3000);

...where the function getyval() uses $.get():

function getyval(){
  $.get('testget.php', function(output){
    alert(parseFloat(output));
  });
};

My testget.php file:

<?php
session_start();
$db = $_SESSION['monitorId'];
$table = $_SESSION['tableId'];
$split_table = explode("_", $table);
$param = $split_table[1];

$dbcon = mysqli_connect("localhost","root","",$db);
$query = "SELECT * FROM ".$table." ORDER BY datetime DESC LIMIT 1";
$lastentry = mysqli_query($dbcon, $query) or die('error reading table');
$row = mysqli_fetch_array($lastentry, MYSQLI_ASSOC);
$yval = $row[$param];
echo $yval;    
?>

This works all well and good at "alerting" the last value every 3 seconds but when I try to assign the variable y to this result, it does not work. For example, if I change the getyval() function to:

function getyval(){
  $.get('testget.php', function(output){
    return parseFloat(output);
  });
};

Thanks in advance!

Answer 1

If you use just return, it'll return from anonymous function, not from getyval(). Second problem is that getyval() is asynchronous (you'll not receive value just after calling). You have to call that function earlier, save its result somewhere and then use it.

var yValue = null;
function getyval(){
    $.get('testget.php', function(output){
      yValue = parseFloat(output);
    });
 };