How to insert data and update data in object-oriented MySQLi prepare statements?

First of all, you must indicate the object-oriented connection type (it could be a procedure type) but it is not good to mix the types.

$con= new mysqli("localhost", "my_user", "my_password", "midatabase");

Second, you should use prepared statements and avoid concatenating values ​​in your queries as this is a big security issue. After this, there would be no reason to use mysqli_real_escape_string()it since it would not be necessary.

The queryof updatewould be:

$query = "UPDATE tbl_employee SET name=?, address=?, gender= ?,
         designation = ?, age = ?  WHERE id= ? " ;

For himInsert

$query = "INSERT INTO tbl_employee(name, address, gender, designation, age)  
           VALUES(?, ?, ?, ?, ?); "; 

Then, depending on the query that you want to execute, use the binding of the values ​​that arrive by POST, for the syntax check the documentation of bind_param() , this after preparing the statement, is placed in a ifbecause sometimes the prepare of the statement returns falsefor some error in the query.

if ($stmt = $con->prepare($query)) {
   $stmt->bind_param("sss", $one,$two,$three);
   $stmt->execute();
}

The replacement of if(mysqli_query($con, $query))would be depending on what you want to do, if it were a you insertcould simply validate the return ofexecute()

if($stmt->execute()){
    echo "Insert Exitoso";
}

But if it is an Update , it could not with the previous one since it would not always be validated correctly, for this it could obtain the number of rows affected affected_rows

$stmt->execute();
if($con->affected_rows > 0){
   echo "Update Exitoso";
}

For the last one select, you could do it directly

$query = "SELECT * FROM tbl_employee ORDER BY id DESC"
$result = $con->query($query);

while ($row = $result->fetch_assoc() ) {... }

Possible Final code

if(!empty($_POST))  
 {  
      $output = '';  
      $message = '';  
      $name = $_POST["name"];  
      $address =  $_POST["address"];  
      $gender = $_POST["gender"];  
      $designation = $_POST["designation"];  
      $age = $_POST["age"];  
      // Variable para saber que tipo de consulta se realizará
      $tipo = 0;
      // Variable para validar la correcta ejecución de la query, Insert o Update
      $estado = 0;  
      $id= $_POST["employee_id"];
      if($id != '')   
      {  
        $query = "UPDATE tbl_employee SET name=?, address=?, gender= ?,designation = ?, age = ?  WHERE id= ? " ; 
        $tipo = 1; // seteamos para update
      }  
      else  
      {  
        $query = "INSERT INTO tbl_employee(name, address, gender, designation, age)  
           VALUES(?, ?, ?, ?, ?); ";   
        $tipo =2; //seteamos el tipo para insert
      }  
      // Sí será update 
      if($tipo === 1){
        if ($stmt = $con->prepare($query)) {
           $stmt->bind_param("sssssi", $name,$address,$gender,$designation,$age,$id);
           //Asignamos el valor de la ejecución del update
           $estado = $stmt->execute();
        }
      }
      //Sí sera Insert
      else if($tipo === 2){
        if ($stmt = $con->prepare($query)) {
           $stmt->bind_param("sssss", $name,$address,$gender,$designation,$age);
           $stmt->execute();
           //Asignamos el valor de la ejecución del insert , 
           //si el número afectado de filas es mayor a 0
           $estado = ($con->affected_rows>0);
        }
      }

      // Para saber si se ejecutó correctamete update o insert 
      if($estado){
        $query = "SELECT * FROM tbl_employee ORDER BY id DESC";
        $result = $con->query($query);
        while ($row = $result->fetch_assoc() ) {
            $output .= '  
               <tr>  
                  <td>' . $row["name"] . '</td>  
                  <td><input type="button" name="edit" value="Edit" id="'.$row["id"] .'" class="btn btn-info btn-xs edit_data" /></td>  
                  <td><input type="button" name="view" value="view" id="' . $row["id"] . '" class="btn btn-info btn-xs view_data" /></td>  
             </tr>  
                    '; 
        }
        echo $output;  
      }



 }