Php Error Message On Invalid Input

I am new to php I am trying to print an error message If a person insert anything other then a real number in the field. I hope it make sense here is the code, and please describe

Solution 1:

<?php
$result = "";
 class calculator
 {
   var $a;
   var $b;

   function checkopration($oprator)
   {
       switch($oprator)
       {
         case '+':
           return $this->a + $this->b;
           break;

         case '-':
           return $this->a - $this->b;
           break;

         case '*':
           return $this->a * $this->b;
           break;

         case '/':
           return $this->a / $this->b;
           break;

         default:
           return "Sorry No command found";
       }
   }
   function getresult($a, $b, $c)
   {
       $this->a = $a;
       $this->b = $b;
       return $this->checkopration($c);
   }
 }

$cal = new calculator();
if(isset($_POST['submit']))
{
  $n1=$_POST['n1'];
  $n2=$_POST['n2'];
  $op=$_POST['op'];
  if(is_numeric($n1)&&is_numeric($n2)){
       $result = $cal->getresult($n1,$n2,$op);
  }else{
    $result="<p style='background-color:#FFCCCC'><b>ERROR: </b>Invalid input</p><br>";
  }
}
?>

  <form method="post">
  <table align="center">
     <tr>
       <td><strong><?php echo $result; ?><strong></td>
   </tr>
   <tr>
          <td>Enter 1st Number</td>
    <td><input type="text" name="n1"></td>
</tr>

<tr>
    <td>Enter 2nd Number</td>
    <td><input type="text" name="n2"></td>
</tr>

<tr>
    <td>Select Oprator</td>
    <td><select name="op">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
    </select></td>
</tr>

<tr>
    <td></td>
    <td><input type="submit" name="submit" value="Submit"></td>
</tr>

 </table>
 </form>

Solution 2:

What you are looking for is the is_numeric() function.
Take a look: http://php.net/manual/de/function.is-numeric.php

Solution 3:

using the is_numberic() function in php you can check if the inputed value is a number and if not we can return an error message

<?php
$result = "";
 class calculator
{
var $a;
var $b;

function checkopration($oprator)
{
    switch($oprator)
    {
        case '+':
        return $this->a + $this->b;
        break;

        case '-':
        return $this->a - $this->b;
        break;

        case '*':
        return $this->a * $this->b;
        break;

        case '/':
        return $this->a / $this->b;
        break;

        default:
        return "Sorry No command found";
    }
}
function getresult($a, $b, $c)
{
    $this->a = $a;
    $this->b = $b;
    return $this->checkopration($c);
}
}

$cal = new calculator();
if(isset($_POST['submit']))
{
    if(is_numeric($_POST['n1']) && is_numeric($_POST['n2'])){
        $result = $cal->getresult($_POST['n1'],$_POST['n2'],$_POST['op']);
    }else{
        $result="please only insert numbers into the calculator";
    }
}
?>