在返回到html页面后，如何在表单提交后显示echo？

This will show the echo on a blank white page.

$existsQuery = "select count(*) as count from entry where emailaddress like '".$_POST[emailaddress]."'";
$existsResult = mysqli_query($con, $existsQuery);

if($existsResult->fetch_object()->count > 0)
{
    echo "email already exist";
    /* header('index.html?email=exists'); */
}

I want to show the echo under the submit button on the main html page. Is using the URL a good way and how do I do it?

Is using the URL a good way and how do I do it?

I think that's a good solution, so the code would look like this:

$existsQuery = "select count(*) as count from entry where emailaddress like '".$_POST[emailaddress]."'";
$existsResult = mysqli_query($con, $existsQuery);

if($existsResult->fetch_object()->count > 0)
{
    header('index.html?email=exists');
}

and then you can do this right below the button submit button in your form:

<?= if (isset($_GET["email"]) && $_GET["email"] == "exists") { echo "email already exists"; } ?>

You can use AJAX to do that.

Take a look at this example:

http://www.w3schools.com/ajax/tryit.asp?filename=tryajax_first

You can start with the basics here http://www.w3schools.com/ajax/default.asp and then learn how to use the $_POST method to your specific case: