Checks In PHP are not working - php

Question

I am working in PHP. i have made a form named as Donor.php and a connect it to database. Now I am trying to apply checks in on it in PHP. But their is a problem. As I have applied checks for empty fields in PHP on a form but these checks are not working. Please check out my code. As my work is stuck just because of this problem. My code file is here:
Donor.php
<?php
//error_reporting(0);
if(isset($_POST['submit'])){
$first_name=$_POST['firstname'];
$last_name=$_POST['lastname'];
$Country=$_POST['country'];
$City=$_POST['city'];
$Gender=$_POST['gender'];
$Email=$_POST['email'];
$Password=$_POST['pwd'];
include_once "connectionn.php";
$emailChecker=mysql_real_escape_string($Email);
$sql_email_check=mysql_query("Select Email FROM user WHERE Email='$emailChecker'");
$email_check=mysql_num_rows($sql_email_check);
if((empty($first_name)) ||(empty($last_name)) ||(empty($City)) ||(empty($Gender)) ||(empty($Email)) ||(empty($Password))) {
$errorMsg='We are sorry, but there appears to be a problem with the form you submitted.';
if (empty($first_name)) {
$errorMsg.='$var is either 0, empty, or not set at all';
header('Location: Donor.php');
}
if(empty($last_name)){
$errorMsg.='lastname';
header('Location: Donor.php');
}
if(empty($City)){
$errorMsg.='City';
header('Location: Donor.php');
}
if(empty($Gender)){
$errorMsg.='Gender';
header('Location: Donor.php');
}
if(empty($Email)){
$errorMsg.='email';
header('Location: Donor.php');
}
if(empty($Password)){
$errorMsg.='Password';
echo "$errorMsg.";
header('Location: Donor.php');
}
}else if($email_check>0){
$errorMsg="invalid";
}else{
$sql="INSERT INTO user (User_ID,First_Name, Last_Name, gender, city, Email, Password) VALUES (NULL,'$first_name', '$last_name','$Gender','$City','$Email','$Password')";
$result=mysql_query($sql);
$UserID="SELECT max(User_ID) as usr from user";
$userIDResult=mysql_query($UserID);
if($userIDResult === false)
{
die(mysql_error());
}
while($R=mysql_fetch_array($userIDResult)){
$usrID= $R['usr'];
}
$donor="INSERT INTO donor(User_ID, Country)Values('".$usrID."','$Country')";
$resultdonor=mysql_query($donor);
mysql_close();
header('Location: DonorPro.php');
}
}
?>
<?php
include "Header.php";
//include "registration.php";
?>
<div class="DonorDiv">
<h1>Lets Join:</h1>
<form name="input" action="" method="post" <?php print"$errorMsg"; ?>>
First Name: <input type="text" name="firstname" placeholder="First Name" id="r">
<?php print "$first_name";
// if (!isset($_POST['firstname'])) {
//echo '$var is either 0, empty, or not set at all';
//}
?>
Last Name: <input type="text" name="lastname" placeholder="Last Name" id="u" <?php print "$last_name";?>> <br>
Institution: <input type="text" name="country" placeholder="Institution" id="" <?php print "$Institution";?>>
City: <input type="text" name="city" placeholder="City" id="" <?php print "$City";?>><br>
Country: <input type="text" name="country" placeholder="Country" id="" <?php print "$Country";?>><br>
Gender: <input type="text" name="gender" placeholder="Gender" id="" <?php print "$Gender";?>><br>
Email Address: <input type="Email" name="email" placeholder="Email" id="g" <?php print "$Email";?>><br>
Password:<input type="Password" name="pwd" placeholder="Password" id="v" <?php print"$Password";?>><br>
<input type="submit" src="images/button(9).png" alt="Submit" id="q">
</form>
</div>
<?php include "Footer.php"; ?>

Answer 1

The PHP mysql lib is deprecated, you should consider using myslqi or php PDO instead.
Here is a tutorial
You should also be careful : $first_name and the other variables as they are not defined when you display the form, so you will get warnings.
Anyway, your problem is that this check is always false :
if(isset($_POST['submit'])){
The easiest (but not the best) way to correct that is to add a hidden input in your form :
<input type="hidden" name="hidden">

Answer 2

You have to quit the PHP script after telling the browser to redirect to another page:
header('Location: Donor.php');
exit;
(Besides SQL injection and some other problems.)