I have a button when click it will insert the statement to the database however even without clicking the button the insert statement is already run .why?

```
<?php
header('P3P: CP="NOI ADM DEV PSAi COM NAV OUR OTRo STP IND DEM"');
include_once 'mysqli.connect.php';
include_once 'fbmain.php';

if($me){
    $fbid= $facebook->api('/me');
    $fbme = $fbid['id'];
    $fbName = $fbid['name'] ;  
    $fbEmail = $fbid['email'];
}
    if (isset($_POST)){
        mysqli_query($mysqli, "INSERT INTO members (fbId, name, email) values ('$fbme', '$fbName', '$fbEmail')") or die(mysql_error());
    }
?>
<html>
<form action="" method="post">
    <input type="image" src="../images/buy.png" name="submit" width="60"height="30" />
</form>
</html>

```