Divide by 0 Error in PHP calculator

Solution:

$answer is being evaluated on page load, as dc2 commented, nothing is stopping your code from being executed as soon as you load the page...

$answer = ($valuea - $valueb) / $valuec;

this produces your error when you load the page.

To avoid this you should check for 0 before you do the division:

if($valuec == 0){
    $answer = "TO INFINITY AND BEYOND";
}else{
   //division here
}
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Solution:

So it looks like you check in the first three lines of code to see if the $_POST[] variables are set, but then regardless of whether they are or not, you go ahead and assign $answer a value. So what happens if those values aren't set?

I haven't written PHP in some time, so I may be wrong, but you may want to check $_POST[] for data prior to executing any code whatsoever and if you find nothing has been set, redirect the user to the form that they should have filled out.

Just my $0.02 - let me know if it works.

Solution:

You could change

$answer = ($valuea - $valueb) / $valuec;
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to

if ((isset($valuea))&&(isset($valueb))&&(isset($valuec))){
     $answer = ($valuea - $valueb) / $valuec;
}
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Then you will avoid the calc with empty values

Solution:

I'd take advantage of is_numeric and also test for 0 before dividing like so:

<?php
$valuea='';
$valueb='';
$valuec='';
$answer='';

if (isset($_POST['valuea'])) $valuea = $_POST['valuea'];
if (isset($_POST['valueb'])) $valueb = $_POST['valueb'];
if (isset($_POST['valuec'])) $valuec = $_POST['valuec'];
if(is_numeric($valuea) && is_numeric($valueb) && is_numeric($valuec))
{
  if($valuec=='0')
  {
    $answer ="Error: Dividing by zero";
  }
  else{
    $answer = ($valuea - $valueb) / $valuec;
  }
}

echo <<<_END
<form method='post'>
<table border='0' width='500px' cellpadding='3' cellspacing='1' >
<tr ><td colspan="2"><strong>Stright Line Depreciation</strong></td></tr>
<tr ><td>Enter Cost:</td><td align="center"><input type="text" name="valuea" value="$valuea" /></td></tr>
<tr ><td>End Salvage Value:</td><td align="center"><input type="text" name="valueb" value="$valueb" /></td></tr>
<tr ><td>Enter Estimated Life:</td><td align="center"><input type="text" name="valuec" value="$valuec" /></td></tr>
<tr ><td colspan="2"><input type="submit" value="Calculate" /></td></tr>
_END;

?>
<tr ><td><em>The answer is:</em></td><td align="center"><input type="text" value="<?php echo is_numeric($answer)?round($answer):$answer; ?>"></td></tr>
</table>
</form>
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Here's a phpfiddle and its results.