PHP - 当密码为空时，我的代码不起作用

I'm doing a login page, I'm a beginner in PHP so you may see that my code doesn't look the most effective, but in time I will improve my skills. I've got a problem in the following code, when I'm trying to log in if I leave the password blank, the user may enter without password, but if I'm typing something, even 1 character, the code works. Why does my code not work correctly with a blank password, and how may I fix it?

Here's the code:

<?php
if($_POST["email"] == null || $_POST["password"] == null )
    header("Location:index.php");

$email = $_POST["email"];
$password = $_POST["password"];

/*The Function checks if there's any user if that email - Primiary Key!*/
function UserExist()
{
    global $email,$password;
    $con = mysqli_connect("127.0.0.1:3306", "root", "root", "photoshare");
    if(mysqli_connect_errno($con))
    {
        echo "לא הצליח להתחבר למסד נתונים</br>". mysqli_connect_error();
    }
    else
    {   
        /*result == Checks if the user exits*/
        $result = mysqli_query($con, "SELECT * FROM tbl_users WHERE email='$email'") or die(mysqli_error($con)); //Query which checks if the email exits in the db
        if(!mysqli_num_rows($result))
            header("Location:index.php");
        PasswordExist($password); // checks if both of passwords are equals
    }

    mysqli_close($con);
}

//Checks if both password are equals
function PasswordExist($password)
{
    global $email,$password;

    $con = mysqli_connect("127.0.0.1:3306", "root", "root", "photoshare"); // Connect db
    $query = "SELECT password FROM tbl_users WHERE email='$email'";//Spefic Row and Column !
    $result = mysqli_query($con, $query);
    $realPassword; // The real password from the db
    //The while loop is taking the row which the the destination is does,and take by is column too . it means it's take his x and y.
    while($realPassword = mysqli_fetch_array($result))
    {       
        echo $realPassword[0]; //The Password from db
        if($realPassword[0] != $password)
        {       
            echo 'הסיסמא שגוייה אנא נסו להתחבר שנית<a href="index.php">חזרה לדף הבית</a>'; //Error massage if the passwords are difrrent
        }
    }

    mysqli_close($con);
}

//Gets username nickname
function UserNameDetails()
{
    global $email,$password;
    $con = mysqli_connect("127.0.0.1:3306", "root", "root", "photoshare");
    $query = "SELECT nickname FROM tbl_users WHERE email='$email'";//Spefic Row and Column !
    $result = mysqli_query($con, $query);
    $nickname;
    while($nickname = mysqli_fetch_array($result))
    {       
        echo $nickname[0];
        setcookie("loggerUserNickName", $nickname[0], time()+1800); //Create cookie which save nickname of the user
    }

    mysqli_close($con);
}

//Change login.php form - as reason the user is logged.
function fileLoginPhpChanger()
{
    $file = "login.php";
    $fh = fopen($file, 'w');
    $data =
        '
        <?php 
        if($_COOKIE["loggerUserNickName"]==null)
        header("Location:loggedoutSpeical.php");
        ?>
        <div id="logged">
        שלום, <?php echo $_COOKIE["loggerUserNickName"];?>
        </br>
        <a href="loggedout.php">לניתוק מן המשתמש</a>
        </div>
        ';
    fwrite($fh, $data);
    fclose($fh);
}

UserExist();
UserNameDetails();
fileLoginPhpChanger();

?>