Update form fields after posting with PHP_SELF - php

Question

I am using php_self to submit a form. Once the data has been posted, I want to pass a calculated value to another form field on the same page, original form.
The $title_insurance field stays blank. Any ideas on why? Thanks!
<?php
if(isset($_POST['submit']))
{
$sale_price = $_POST['sale_price']; // posted value
$title_insurance = ($sale_price * 0.00575) + 200;
?>
<script type="text/javascript">
document.getElementById("title_insurance").value='<?php echo $title_insurance ; ?>';
</script>
<?php } ?>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" enctype="multipart/form-data">
<input name="sale_price" type="text" id="sale_price" size="15">
<input name="title_insurance" type="text" id="title_insurance" size="15" value="<?php echo $title_insurance; ?>" />
<input name="submit" type="submit" class="bordered" id="submit" value="Calculate" />
</form>

Answer 1

The submit button is called button, also if you are outputting a javascript to amend the value it need to be run after the DOM has created the element title_insurance.
if(isset($_POST['button']))
{
$sale_price = $_POST['sale_price']; // posted value
$title_insurance = ($sale_price * 0.00575) + 200;
}
?>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" enctype="multipart/form-data">
<input name="sale_price" type="text" id="sale_price" size="15">
<input name="title_insurance" type="text" id="title_insurance" size="15" value="<?php echo $title_insurance; ?>" />
<input name="button" type="submit" class="bordered" id="button" value="Calculate" />
</form>
<script type="text/javascript">
document.getElementById("title_insurance").value='<?php echo $title_insurance ; ?>';
</script>
A better way in this case would be to forget about the javascript as it is unnecessary and do this
// I am assuming you have initialized $title_insurance
// somewhere above here to its default value!!!!
$title_insurance = isset($_POST['button']) ? ($_POST['sale_price'] * 0.00575) + 200 : $title_insurance;
?>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" enctype="multipart/form-data">
<input name="sale_price" type="text" id="sale_price" size="15">
<input name="title_insurance" type="text" id="title_insurance" size="15" value="<?php echo $title_insurance; ?>" />
<input name="button" type="submit" class="bordered" id="button" value="Calculate" />
</form>

Answer 2

You have an extra space in your getElementById parameter:
// VV
document.getElementById("title_insurance ").value='<?php echo $title_insurance ; ?>';

Answer 3

What you want to do is best done by AJAX. The <form> construction is outdated and not useful unless you are transferring the user to another page and sending some data along with it - or, if you are finished getting user data and just want to process what was entered and display a completion message.
If you wish to continue processing on the same page, then AJAX is the way to go. And the best way to use AJAX is to have a separate processor file (PHP) that receives the data, processes it, and sends it back.
To convert a <form> construct to AJAX, you really just need to remove the <form></form> tags and convert the submit button from type="submit" to type="button" id="mybutton", and use the IDs on the button and on the other elements to grab the data they contain and feed them to the AJAX code block. The examples in the link at bottom shows what you need to know - they are simple, helpful examples.
To conserve resources, you can use the same PHP processor page for multiple AJAX requests -- just send a variable (eg. 'request=save_to_db&first_name=bob&last_name=jones', ) and test for what "request" is received, that will determine what your PHP processor file does and echoes back.
This post, and the examples it contains, will help.

Answer 4

try this first
In you coding you missed this $_POST['button']
and
<?php
if(isset($_POST['button']))
{
$sale_price = $_POST['sale_price']; // posted value
$title_insurance = ($sale_price * 0.00575) + 200;
?>
<script type="text/javascript">
document.getElementById("title_insurance ").value='<?php echo $title_insurance ; ?>';
</script>
<?php } ?>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" enctype="multipart/form-data">
<input name="sale_price" type="text" id="sale_price" size="15">
<input name="title_insurance" type="text" id="title_insurance" size="15" value="<?php echo $title_insurance; ?>" />
<input name="button" type="submit" class="bordered" id="button" value="Calculate" />
</form>
and also refer this FIDDLE it will more helpful to you..