我有一個表單，一旦從主下拉列表中進行選擇，它就會分成兩個不同的下拉列表。問題是，當我提交表單時，我需要從每個子下拉列表中進行選擇才能提交表單。如果未使用該變量，我該如何讓 PHP 代碼忽略該變量？例如，我希望能夠選擇“擊劍”>“colorbond”。但我還需要從內部民事>任何中進行選擇。為了能夠提交表單。<?php if(isset($_POST['submit'])){  $firstName = $_POST['first-name'];  $lastName = $_POST['last-name'];  $mailFrom = $_POST['email'];  $tel = $_POST['tel'];  $streetAddress = $_POST['street-address'];  $suburb = $_POST['suburb'];  $jobType = $_POST['job-type'];  $subJobTypeCivil = $_POST['internal-civil'];  $subJobTypeFencing = $_POST['fencing-job'];  $subject = "$jobType $firstName $lastName $suburb";    echo ''.$jobType;  $approximateMeterage = $_POST['approx-meterage'];  $message = $_POST['message'];// workout how to files to upload////////////////    $mailTo = "[email protected]";  $headers = ("WEBQ $jobType $firstName $lastName $suburb");  $txt = "New ".$jobType." enquiry. \n\n"."Customer Name: ".$firstName." ".$lastName."\n"."Email: ".$mailFrom."\n"."Phone: ".$tel."\n"."Address: ".$streetAddress.", ".$suburb."\n"."Job Type: ".$jobType."\n".   "Civil Job Type: ". $subJobTypeCivil."\n"."Fencing JobType:" .   $subJobTypeFencing .      "\n"."Approximate Meterage: ".$approximateMeterage."\n\n"."Message: ".$message;  mail($mailTo, $subject, $txt, $headers);  header("Location: index.html");  }?>function navHamburger() { var x = document.getElementById("nav-links"); if (x.style.display === "block") {  x.style.display = "none"; } else {  x.style.display = "block"; }}// CONTACT US FORM ////////////////////////////////////////////////////////////////////$("#job-type").change(function(){ if ($(this).val() == "internals-civil") {  $('#telecom-internal-civil').show();  $('#internal-civil').attr('required', '');  $('#internal-civil').attr('data-error', 'This felid is required'); } else {  $('#telecom-internal-civil').hide();  $('#internal-civil').removeAttr('required');  $('#internal-civil').removeAttr('data-error'); }});