isset（$ _POST ['submit']）总是返回false（我没有忘记name =“submit”）[关闭]

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).

EDIT - The jQuery:

handler : function(){
    var jqxhr = $.ajax({
        type: 'POST',
        url: '../php/enquiry.php',
        data: $('#page-footer_enquiry-form').serialize() + "&js=true",
        success : function(res) {
            console.log(res);
        }
    });
},

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

If you have a look at documentation of serialize() https://api.jquery.com/serialize/ you will find the following:

No submit button value is serialized since the form was not submitted using a button.

I think that causes your problem.