Using a combo of PHP and HTML to input values and then output values

Question

I am new to PHP and have just started learning it. I may have gotten in over my head. I am trying to have a user input a buy value and a sell value in an html form, and then have php perform mathematical operations on those values, and finally, output the value calculated by the math operations.

<html>
<head>
 <title>PHP Test</title>
</head>
<body>

<form action="" method="post">
 Buy Price: <input type="number" name="buyprice" value="<?php echo 
 $buyprice;?>">
 Sell Price: <input type="number" name="sellprice" value="<?php echo 
 $sellprice;?>">
 <input type="submit" name="submit" value="Calculate" />
</form>

<?PHP

  $buyprice = $_POST['buyprice'];
  $sellprice = $_POST['sellprice'];
  $tax = 0.95;
  $profit = 0;

  if (isset($_POST['submit'])) { //to check if the form was submitted
  $profit = (($sellprice * $tax) - $buyprice);
  print $profit;
 }


?>


</body>
</html>

This is my attempt at it, I would appreciate any tips or ideas on how to accomplish what I want to do. Basically, the user inputs two values into the form, the php needs to use those values as variables and perform mathematical operations with them, and then I need it to output the calculated value to the page. The calculation should not be performed until the user presses the calculate button.

I'm also curious if there is a way to prevent the user from adding numbers that are negative in value (min max?). I was also wondering if there was a way to remove the arrows from the input boxes in the form.

Thanks.

In what way is your code currently not working? What exactly is the problem that you're experiencing? — David
– David, Commented Sep 7, 2017 at 14:24
Yes there are a lot of ways to check for min and maximum values. Though not directly in html attributes. But you can perform checks in Javascript, Php, etc. — Mark Baijens
– Mark Baijens, Commented Sep 7, 2017 at 14:25
(on a side note, your $profit value should be inside html tags <p> for example. Most browsers will show it anyway, but it's not according to w3 standards. — Mark Baijens
– Mark Baijens, Commented Sep 7, 2017 at 14:28
It seemingly takes my input in the input boxes; however, I cannot tell if it is calculating anything, and it will not output anything to my webpage at all when I press the calculate button. I'm not sure if this output problem is related to an issue with my math, or if I am just missing something. — FrenchJamesMilner
– FrenchJamesMilner, Commented Sep 7, 2017 at 14:37

IsThisJavascript · Accepted Answer · 2017-09-07 14:37:56Z

Your code works, you just messed up on where to put your PHP.

<?PHP

  $buyprice = $_POST['buyprice'];
  $sellprice = $_POST['sellprice'];
  $tax = 0.95;
  $profit = 0;

  if (isset($_POST['submit'])) { //to check if the form was submitted
  $profit = (($sellprice * $tax) - $buyprice);
  print $profit;
 }


?>

<html>
<head>
 <title>PHP Test</title>
</head>
<body>

<form action="" method="post">
 Buy Price: <input type="number" name="buyprice" value="<?php echo 
 $buyprice;?>">
 Sell Price: <input type="number" name="sellprice" value="<?php echo 
 $sellprice;?>">
 <input type="submit" name="submit" value="Calculate" />
</form>

</body>
</html>

To remove the arrows on the right, you can swap out <input type="number" for <input type="textbox".

To stop users from inputting negative numbers, you can use the pattern element for this. <input type="textbox" pattern="^[1-9][0-9]*$"

^[1-9][0-9]*$ is regex for any number above 0, however it won't accept a decimal.

If you can live with the arrows, then you can set a min property to stop input going below 0.
<input type="number" min="0"

NOTE: Even tho you are stopping users from inputting a value > 0. A malicious user may still send a fake $_POST value to your page. If you're doing anything sensitive with $_POST/$_GET/$_REQUEST you cannot trust it.

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