PHP determine if request contains POST form submission

I have this form for resetting passwords:

<form action = "resetpwd.inc.php" method = "post" class = "form">
    <div class = "input-group">
        <input type = "text" name = "email" placeholder = "Enter your email:">
        <span class = "icon">&#9993;</span>
    </div>
    <div class = "input-group">
        <button class = "submitbtn" type = "submit">Reset Password</button>
    </div>
</form>

I also have a resetpwd.inc.php file with the following code that handles the form submission:

if(isset($_POST["submitbtn"]))

My goal is to have this PHP code execute when I submit the form, however it isn’t doing that and it’s just refreshing the page.