set value of input field by php variable's value

One way to do it will be to move all the php code above the HTML, copy the result to a variable and then add the result in the <input> tag.
Try this - 

<?php
//Adding the php to the top.
if(isset($_POST['submit']))
{
    $value1=$_POST['value1'];
    $value2=$_POST['value2'];
    $sign=$_POST['sign'];
    ...
        //Adding to $result variable
    if($sign=='-') {
      $result = $value1-$value2;
    }
    //Rest of your code...
}
?>
<html>
<!--Rest of your tags...-->
Result:<br><input type"text" name="result" value = "<?php echo (isset($result))?$result:'';?>">

Tags:

Html

Php

Variables

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