op.php
<?php




    if($_POST["num1"]=="" || $_POST["num2"]=="")
    {
        if($_POST["num1"]=="")
            echo "Please enter Number 1 <br/>";
        if($_POST["num2"]=="")
            echo "Please enter Number 2 <br/>";
    }
    else
        if($_POST["num2"]==0)
    {

            echo "Number 2 should not be Zero. this is divided by zero error.";
    }
    else
    {

    echo $_POST["num1"] . " " . $_POST["Operation"] . " " . $_POST["num2"] . " = ";

    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "calc";

    // Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);
    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }

    $sql = "SELECT result FROM operations where number1=".$_POST["num1"]." and number2=".$_POST["num2"]." and operation='".$_POST["Operation"]."'" ;
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        // output data of each row
        while($row = $result->fetch_assoc()) {
            echo $row["result"];
        }
    echo "<br/> Result is displayed from database";
    }
    else {
        $resultVal=0;

        switch($_POST["Operation"])
        {
            case "-":
               echo $resultVal=$_POST["num1"] - $_POST["num2"];
                break;
            case "*":
               echo $resultVal=$_POST["num1"] * $_POST["num2"];
               break;
            case "/":
               echo $resultVal=$_POST["num1"] / $_POST["num2"];
               break;
            default:
               echo $resultVal=$_POST["num1"] + $_POST["num2"];
               break;
        }

        $sql = "INSERT INTO operations('number1','number2','operation','result')
                VALUES (".$_POST["num1"].",".$_POST["num2"].",'".$_POST["Operation"]."',".$resultVal.");";
        $result = $conn->query($sql);
        echo "<br/> This is first time operation is queried.";
    }
    $conn->close();



    }



?>