isset( $_POST['submit'] ) is always returning false (I haven't forgotten name="submit")

I've the following HTML form:

<form class="page-footer_enquiry-form" id="page-footer_enquiry-form" method="post" autocomplete="on" action="../php/enquiry.php">
    <h2 class="page-footer_enquiry-form-title">Enquiry</h2>
    <!-- Name -->
    <div class="page-footer_enquiry-form-name">
        <label for="footer-enquiry-name">
            <span class="page-footer_enquiry-form-label">Full Name</span>
            <input type="text" id="footer-enquiry-name" name="fullname">
        </label>
    </div>
    <!-- Email -->
    <div class="page-footer_enquiry-form-email">
        <label for="footer-enquiry-email">
            <span class="page-footer_enquiry-form-label">Email Address</span>
            <input type="email" id="footer-enquiry-email" name="email">
        </label>
    </div>
    <!-- Message -->
    <div class="page-footer_enquiry-form-message">
        <label for="footer-enquiry-message">
            <span class="page-footer_enquiry-form-label">Message</span>
            <textarea name="message" id="footer-enquiry-message" rows="10" cols="50"></textarea>
        </label>
    </div>
    <input name="submit" type="submit" value="Send Message" class="button-dark">
    <p class="page-footer_enquiry-form-dialog"></p>
</form>

Which is "handled" by the following PHP:

if( isset( $_POST['submit'] ) ){
    echo 'first';
} else {
    echo 'second';
};

Whenever I press the submit button, I'm getting 'second' (logged to the console via js).

I fear it'll be something obvious to somebody else, but I just can't identify the problem. Help, please.