PHP - Consulta con campos inputs date

<?php

include "config.php";

$filename = 'employee_'.time().'.csv';

// POST values

$from_date = $_POST['from_date'];

$to_date = $_POST['to_date'];

// Select query

$query = "SELECT * FROM employee ORDER BY id asc";

if(isset($_POST['from_date']) && isset($_POST['to_date'])){

   $query = "SELECT * FROM employee where date_of_joining between '".$from_date."' and '".$to_date."' ORDER BY id asc";

}

$result = mysqli_query($con,$query);

$employee_arr = array();

$file = fopen($filename,"w");

$employee_arr = array("id","Employee Name","Salary","Gender","City","Email","Date of Joining");

while($row = mysqli_fetch_assoc($result)){

   $id = $row['id'];

   $emp_name = $row['emp_name'];

   $salary = $row['salary'];

   $gender = $row['gender'];

   $city = $row['city'];

   $email = $row['email'];

   $date_of_joining = $row['date_of_joining'];

   // Write to file

   $employee_arr = array($id,$emp_name,$salary,$gender,$city,$email,$date_of_joining);

   fputcsv($file,$employee_arr);

}

fclose($file);

// download

header("Content-Description: File Transfer");

header("Content-Disposition: attachment; filename=$filename");

header("Content-Type: application/csv; ");

readfile($filename);

// deleting file

unlink($filename);

exit();

mysqli_num_rows($result);

if (mysqli_num_rows($result)>0)

  {

    // tu codigo ...

  }

  else

  {

   echo '<script language="javascript">alert("bla bla bla");</script>';

  }