Try this, if you want to use a PHP only solution to your problem. 
<?php
function submit_form(){
    $host = "localhost";
    $user = "user";
    $password = "password";
    $database = "database";   
    $firstname  = filter_var($_POST['firstname'], FILTER_SANITIZE_STRING); 
    $lastname   = filter_var($_POST['lastname'], FILTER_SANITIZE_STRING); 
    $email  = filter_var(filter_var($_POST['email'],FILTER_SANITIZE_EMAIL),FILTER_VALIDATE_EMAIL);
    // open connection to database
    $link = mysqli_connect($host, $user, $password, $database);
        IF (!$link){
            echo ("Unable to connect to database!");
        }
        ELSE {
           //INSERT VALUES INTO DATABASE
           $query = "INSERT INTO users (firstname,lastname,email) 
VALUES('".$firstname."', '".$lastname."', '".$email."'";
           return mysqli_query($link,$query);
        }
//close connection to database
        mysqli_close($link);
    }
$form = <<<EODuserform
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
    <head>
        <title>Form</title>
    </head>
        <form action="{$_SERVER['PHP_SELF']}" method="POST" name="userform">
            <label for='first'>First Name:</label></br>
            <input type="text" name="firstname" id="first" maxlength="25" tabindex='1' VALUE="firstname" /></br>
            <label for='first'>Last Name:</label></br>
            <input type="text" name="lastname" id='lastname' maxlength="25" tabindex='2' VALUE="lastname" /></br>
            <label for='email'>E-mail:</label></br>
            <input type="text" name="email" id='email' maxlength="100" tabindex='3' VALUE="email" /></br>
            <input type="submit" class='button' name="submit" value="submit" tabindex='4' />
            </form>
    </body>
</html>
EODuserform;
IF(!IsSet($_POST['submit'])){ // Check if form is not send, if not display empty form.
    echo $form;
}
ELSE{
// in the case you want to send something to the database use 
submit_form();
echo ('Thanks for submitting your form');
}
?>
It is a very basic script you can find a similar script on this page: Form not saving to database