calculator database using php

Hi all this post explains implementation of calculator database in php
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<TITLE>PHP Calculator v1</TITLE>
<form name="calc" action="?page=calc" method="POST"> <!--implementing pagination with the page name appended to url -->
Number 1: <input type=text name=value1><br>
Number 2: <input type=text name=value2><br>
Operation: <input type="submit" name=oper value="add"/> <input type="submit" name=oper value="subtract"/>
<input type="submit" name=oper value="divide"/> <input type="submit" name=oper value="multiply"/><br>
</form>

<?php
if($_SERVER['REQUEST_METHOD'] === 'POST')//checking if the form method is post
{
$page = $_GET['page'];  
// Defining the "calc" class
class calc {
var $number1;  //variable declaration
var $number2;  
function add($number1,$number2)  {
$con=db();       //creating connection variable            
$result =$number1 + $number2;
echo("The sum of $number1 and $number2 is $result<br><br>");
echo("$number1 + $number2 = $result");
mysqli_query($con,"insert into plus values('$number1 + $number2',$result)");
exit;            
}  
function subtract($number1,$number2)  {
$con=db();                  
$result =$number1 - $number2;
echo("The difference of $number1 and $number2 is $result<br><br>");
echo("$number1 &#045 $number2 = $result");
mysqli_query($con,"insert into plus values('$number1 - $number2',$result)");
exit;          
 }  
function divide($number1,$number2)  {
$con=db();                  
$result =$number1 / $number2;
echo("$number1 divided by $number2 is $result<br><br>");
echo("$number1 &divide $number2 = $result");
mysqli_query($con,"insert into plus values('$number1 / $number2',$result)");
exit;          
 }  
function multiply($number1,$number2)  {
$con=db();                  
$result =$number1 * $number2;
echo("The product of $number1 and $number2 is $result<br><br>");
echo("$number1 x $number2 = $result");
mysqli_query($con,"insert into plus values('$number1 * $number2',$result)");
exit;            
}
}
$calc = new calc();
 ?>

<?php
if($page == "calc"){
$n1 = $_POST['value1'];
$n2 = $_POST['value2'];
$number1=abs(ltrim($n1,''));
$number2=abs(ltrim($n2,''));
$oper = $_POST['oper'];
function db () {
static $con;
if ($con===NULL){        
$con = mysqli_connect ("localhost", "root", "", "plus");
     }
return $con;
 }
if(!$number1){
echo("You must enter number 1!");
exit;    
 }
if(!$number2){
echo("You must enter number 2!");
exit;      
}
if(!$oper){
echo("You must select an operation to do with the numbers!");
exit;      
}
if(!preg_match("/[0-9]/", $number1)){
echo("Number 1 MUST be numbers!");
exit;      
}
if(!preg_match("/[0-9]/", $number2)){
echo("Number 2 MUST be numbers!");
exit;      
}
if($oper == "add"){          
$con=db();
$query = mysqli_query($con,"SELECT * FROM plus WHERE exp='$number1 + $number2'");
if (!$query) {
        echo 'MySQL Error: ' . mysqli_error();
        exit;
    }
$numrows = mysqli_num_rows($query);
if ($numrows == 0) {
$calc->add($number1,$number2);
      }

}
if($oper == "subtract"){
$con=db();
$query = mysqli_query($con,"SELECT * FROM plus WHERE exp='$number1 - $number2'");
$numrows = mysqli_num_rows($query);
if ($numrows == 0) {        
$calc->subtract($number1,$number2);
      } }
if($oper == "divide"){
$con=db();
$query = mysqli_query($con,"SELECT * FROM plus WHERE exp='$number1 / $number2'");
$numrows = mysqli_num_rows($query);
if ($numrows == 0) {  
          $calc->divide($number1,$number2);    
 }
}
 if($oper == "multiply"){
$con=db();
$query = mysqli_query($con,"SELECT * FROM plus WHERE exp='$number1 * $number2'");
$numrows = mysqli_num_rows($query);
if ($numrows == 0) {  
          $calc->multiply($number1,$number2);       } }
while ($row = mysqli_fetch_assoc($query)) {
print("Accessed from database.......<br>");
print($row['exp'] . " = " . $row['res']);
 }    
}
}
?>

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These are the output screens cross check with xml file:

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