Yesterday Tarquin macey asked me how can we change the value of the textbox based using Ajax and PHP based on the changing value of the dropdown list.Today, I’ve come up with a solution from him.In this post, you’ll see how to get the currency code of a country from PHP using Ajax and this currency code will replace the value of textbox each time the dropdown list changes.

Writing Code for changing textbox value from dropdown list using Ajax and PHP

After looking at the above demo, let’s start writing code for the changing the currency code value in the textbox form Ajax using PHP when you changes the country from the dropdown list.

HTML Code

<select name="country" onChange="getCurrencyCode('find_ccode.php?country='+this.value)">
 <option value="">Select Country</option>
        <option value="1">USA</option>
        <option value="2">UK</option>
        <option value="3">Nepal</option>
</select>
<input type="text" name="cur_code" id="cur_code" >

As you can in the above code, there are two main components one dropdown list whose name is country and contains the list country in it.The JavaScript function getCurrencyCode() is called when user change value in the list. Note down the name and id of textbox which will have the currency code fetched from Ajax.

JavaScript Code for changing textbox value without refreshing the page

function getCurrencyCode(strURL)
{
  var req = getXMLHTTP();
  if (req)
  {
        //function to be called when state is changed
        req.onreadystatechange = function()
        {
          //when state is completed i.e 4
          if (req.readyState == 4)
          {
                // only if http status is "OK"
                if (req.status == 200)
                {
                        document.getElementById('cur_code').value=req.responseText;
                }
                else
                {
                        alert("There was a problem while using XMLHTTP:\n" + req.statusText);
                }
          }
        }
        req.open("GET", strURL, true);
        req.send(null);
  }
}

In the first line of the above code, the XMLHTTPRequest object is created using getXMLHTTP() function. To look at the the structure of this function, take a look at this old post change the value of dropdown list using Ajax and PHP . After checking the appropriate value of readystate(4 means completed) and status(200 means ok) property of XMLHTTPRequest object, the value of textbox is replaced with the returned value from PHP using Ajax . The response ,which is can be accessed via req.responseText property, is written to textbox via the value property of textbox.

PHP code for changing the value of textbox using Ajax and PHP

<?php
$country=$_REQUEST['country'];
switch($country)
{
        case "1" :
                echo "USD";
                break;
        case "2" :
                echo "GBP";
                break;
        case "3" :
                echo "NPR";
                break;
}
?>

As you can see the above PHP code, it is fairly simple which just print the currency code value according to the name of country. The value which is in the echo statement is going to returned from PHP via Ajax.

Download Full Source Code

The problem of precision lost in floating point number in PHP haunted me for about 10 minutes yesterday but I quickly figured out the problem and solved it as I was aware of this problem. But, I would like to post here it here so that many others PHP programmers, who are not aware with this kind of problem ,get solution easily and don’t get stuck with it.

Calculation problem floating Point number in PHP

First of all, let me show you a small piece of example of the PHP code which I was working on.

$no_of_time=(0.60-0.55)*100;
var_dump($no_of_time); //displays float(5)
for($i=1;$i<=$no_of_time;$i++)
{
   echo $i;
}

In the above code,  the line var_dump($no_of_time); displays float(5). Ok, this is a expected result. But what about the loop, which runs just for 4 time. The above loop runs just for four time although var_dump () outputs that it is 5. Yes, it is 5 but with lost precision and that why never ever trust on the regular arithematic calculation with the floating point numbers in PHP. It always gives the wrong result because of the problem of lost precision. You can also see this warning in the PHP’s website.

Solving the problem of floating point precision in PHP

To solve this problem in floating point number, you need the function which calculates and support the calculation of floating point number of any precision. And, I recommend you to use BC Math functions, the binary calculator, of PHP for calculations  of floating point number to prevent yourself from the problem of precision lost. But note down that, these functions always returns calculated output as string.Now, let’s try to solve the problem of above code rewrite it using bcsub and bcmul functions of PHP.

$no_of_time=bcmul(bcsub(0.60,0.55,2),100);
var_dump($no_of_time); //display string(1) "5"
for($i=1;$i<=$no_of_time;$i++)
{
   echo $i;
}

Now the above, loop runs 5 times without any problem.