I need to get work the code where Ajax passes data to php.

Below is script of file ___01.php
There is drop down menu with variables No and Yes. Default value is No.
If i do not change drop down menu variable, script works ok. When I change default value, script does not work (I press Submit and nothing happens).

What is wrong with the code? I suppose problem is related with JavaScript... but do not understand where

Code:
<html>
<head>
<script language="JavaScript" type="text/javascript">
function ajax_post(){
    // Create our XMLHttpRequest object
    if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
var hr = new XMLHttpRequest();
}
else
{// code for IE6, IE5
var hr = new ActiveXObject("Microsoft.XMLHTTP");
}
    // Create some variables we need to send to our PHP file
    var url = "___02.php";
    var fn = document.getElementById("first_name").value;
    var ln = document.getElementById("last_name").value;
	var aphr = document.getElementById("apply_hourly_rate").value;
	var sabt = document.getElementById("salary_before_taxes").value;
    var vars = "firstname="+fn+"&lastname="+ln+"&apply_hourly_rate="+aphr+"&salary_before_taxes="+sabt;
    hr.open("POST", url, true);
    // Set content type header information for sending url encoded variables in the request
    hr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    // Access the onreadystatechange event for the XMLHttpRequest object
    hr.onreadystatechange = function() {
	    if(hr.readyState == 4 && hr.status == 200) {
		    var return_data = hr.responseText;
			document.getElementById("status").innerHTML = return_data;
	    }
    }
    // Send the data to PHP now... and wait for response to update the status div
    hr.send(vars); // Actually execute the request
    document.getElementById("status").innerHTML = "processing...";
}
</script>

<script type="text/javascript">
function hourly_rate(str)
{
if (str=="")
  {
  document.getElementById("hourly_rate").innerHTML="";
  return;
  }
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("hourly_rate").innerHTML=xmlhttp.responseText;
	}
  }
xmlhttp.open("GET","___02.php?hourly_rate="+str,true);
xmlhttp.send();
}
</script>



</head>
<body>
Your First Name: <input id="first_name" name="first_name" type="text" /> 
<br /><br />
Your Last Name: <input id="last_name" name="last_name" type="text" />
<br /><br />


<div id="calculator-left">Hourly rate?</div>
<div id="calculator-middle">
<select name="apply_hourly_rate" id="apply_hourly_rate" onChange="hourly_rate(this.value);" style="margin-bottom:3px; width:70px">
<option value="1">No</option>
<option value="2">Yes</option>
</select>     
</div>
<div class="clear">&nbsp;</div>

<div id="hourly_rate">
Salary before taxes:

<br>
<input type = "text" name = "salary_before_taxes" id = "salary_before_taxes" value="<?php echo $_POST['salary_before_taxes']; ?>" maxlength = "15" size="9"></input>
<br>


<input name="myBtn" type="submit" value="Submit Data" onClick="javascript:ajax_post();">
<br /><br />
<div id="status"></div>



</body>
</html>
second file ___02.php

Code:
<?php 

echo $_POST['firstname'] . ' firstname<br> ' . $_POST['lastname'] . 'lastname <br>'  . $_POST['apply_hourly_rate'] . 'apply_hourly_rate<br>' . $_POST['salary_before_taxes'] . 'salary_before_taxes';


if ($hourly_rate==1) {echo '
Salary before taxes:
<br>
<input type = "text" name="salary_before_taxes" id="salary_before_taxes" maxlength = "15" onkeyup="this.value = this.value.replace(/,/g,'.')" size="9"></input>
<br>
<input name="myBtn" type="submit" value="Submit Data" onClick="javascript:ajax_post();">
';}
if ($hourly_rate==2) {echo '
<br>
Number of hours 
<br>
<input type = "text" name="hours_one" id="hours_one" maxlength = "15" size="9"></input>
<br>
Payment for one hour
<br>
<input type = "text" name="pay_rate_one" id="pay_rate_one" maxlength = "15" size="9"></input>
<br>
<input name="myBtn" type="submit" value="Submit Data" onClick="javascript:ajax_post();">

';}


?>