not able to add data from web page to database - php

Question

this is my form .html file getting connected to adddatabase.php page .having firstname and secondname
<form action="addtodatabase.php" method="post">
<div class="container">
<form class="form-inline">
<fieldset>
<legend>Security Department User Registration</legend>
<div class="form-group">
<label for="Firstname">First Name</label>
<input type="text" class="form-control" id="Firstname" name="firstname" placeholder="Text input"><br/>
</div>
<div class="form-group">
<label for="Secondname">Second Name</label>
<input type="text" class="form-control" id="Secondname" name="secondname" placeholder="Text input"><br/>
</div>
</form>
<button type="submit" class="btn btn-default">Submit</button>
</form>
addtodatabase.php page as follows .
if (isset($_POST)) {
$firstname = isset($_POST['firstname']) ? $_POST['firstname'] : '';
$secondname = isset($_POST['secondname']) ? $_POST['secondname'] : '';
echo 'Your first name is ' .$firstname. '<br>';
echo 'Your second name is ' .$secondname. '<br>';
}
mysqli_query($connect,"INSERT INTO form_details(firstname,secondname)
VALUES('$firstname','$secondname')");

Answer 1

You have form inside a form which is wrong. Do it like this:
<form method="POST" action="addtodatabase.php" >
<!-- Your inputs -->
</form>
Also, put your submit button inside the form:
<form method="POST" action="addtodatabase.php" >
<!-- Your inputs -->
<input type="submit" value="Submit" />
</form>
Or if the submit button is outside, set the form attribute of the button to the id of the form:
<form id="myForm" method="POST" action="addtodatabase.php" >
<!-- Your inputs -->
</form>
<input type="submit" value="Submit" form="myForm" />