PHP Form Validation To Add Class

See my StackOverflow post at https://stackoverflow.com/questions/62107649/add-a-class-to-an-input-using-php

HTML

<!DOCTYPE HTML>
<html lang="en-US">

  <head>
    <meta charset="UTF-8" name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Apply</title>
    <link rel="icon" href="../../Images/Used/Favicon.png" />
    <link rel="stylesheet" type="text/css" href="Apply.css" />
    <link rel="stylesheet" href="../../Other/Font Awesome/fontawesome-free-5.13.0-web/css/all.css" type="text/css" />
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
  </head>

  <body>

    <nav id="navbar">

      <a class="apply activeNav" href="../Apply/Apply.html" title="Apply">Apply</a>
      <a class="home" href="../../index.html" title="Home">Home</a>

      <a class="logoText" href="../../index.html" title="CompRepair Logo" alt="CompRepair Logo">
        <img class="logoPic" src="../../Images/Used/CompRepairLogo.png" />
      </a>

      <a class="signInText" href="../Account/Sign In/SignIn.html" title="Sign In">
        <img class="signInPic" src="../../Images/Used/SignIn.png">
      </a>
      <div class="dropdown">
        <button class="dropbtn" title="Legalities">Legalities
          <i class="fa fa-caret-down"></i>
        </button>
        <div id="myDropdown" class="dropdown-content">
          <a href="#">Terms of Service</a>
          <a href="#">Privacy Policy</a>
        </div>
      </div>

      <a class="contact" href="../Contact/Contact.html" title="Contact">Contact</a>

    </nav>

    <div id="banner">
      <h2 id="bannerText">Apply For A Job At CompRepair</h2>
    </div>


    <div id="formIntroParent">
      <p class="formIntro">Your application is sent through secure connections.</p>
      <p class="formIntro">Have questions about applying? <a href="../Contact/Contact.html" target="_blank">Contact
          us!</a></p>
    </div>

    <form class="applyForm" name="applyForm" action="Apply.php" id="applyForm" method="POST">
      <fieldset class="formSection">
        <h4>Personal Information</h4>
       ...

CSS

* {
  box-sizing: border-box;
  outline: none;
}

html,
body {
  width: 100%;
  height: 100%;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: "Segoe UI", Calibri, "Helvetica Neue", Helvetica, Arial, sans-serif;
  background: #f6f6f6;
  text-align: center;
}


#navbar {
  width: 100%;
  height: 50px;
  background-color: #f1f1f1;
  padding-bottom: 5px;
}

#navbar a {
  height: 50px;
  float: left;
  display: block;
  color: black;
  text-align: center;
  padding: 15px 15px;
  margin-right: 0.5%;
  text-decoration: none;
  font-weight: lighter;
  font-size: 17px;
  transition: color 0.4s ease-in-out;
}

#navbar a:hover {
  cursor: pointer;
  color: gray;
  transition: color 0.4s ease-in-out;
}

#navbar button {
  height: 50px;
  float: left;
  display: block;
  color: black;
  text-align: center;
  padding: 10px 15px;
  text-decoration: none;
}

#navbar .openbtnSmall {
  display: none;
}

#navbar button:hover {
  cursor: pointer;
  color: gray;
}

#navbar button:focus {
  outline: 0;
}

#navbar a.signInText {
  float: right;
  opacity: 0.5;
  transition: opacity 0.4s ease;
}

#navbar a.signInText:hover {
  border: none;
  opacity: 1;
  transition: opacity 0.4s ease;
}

#navbar a img.signInPic {
  max-height: 30px;
  margin-top: -3px;
  text-align: center;
}

#navbar a.logoText {
  float: none;
  display: block;
  position: absolute;
  left: calc(50% - 80px);
  opacity: 0.5;
  transition: opacity 0.4s ease;
}

#navbar a.logoText:hover {
  border-bottom: none;
  opacity: 1;
  transition: opacity 0.4s ease;
}

#navbar a img.logoPic {
  max-height: 30px;
  margin-top: -3px;
  text-align: center;
  position: absolute;
}

#navbar a.activeNav {
  font-weight: bold;
}

.dropdown {
  float: right;
  overflow: hidden;
}

.dropdown .fa-caret-up {
  display: none;
}

.dropdown .dropbtn {
  font-size: 16px;
  border: none;
  outline: none;
  color: black;
  padding: 14px 16px;
  background-color: inherit;
  font-family: inherit;
  margin: 0;
 ...

JavaScript

let form = document.getElementById("applyForm");
function handleForm(event) { event.preventDefault(); }
form.addEventListener('submit', handleForm);

// On click of the text, show/hide the phone number requirements
function showPhoneEx() {
    let phNumRequire = document.getElementById('phNumRequire');
    let phNumReveal = document.getElementById('phNumReveal');

    if (phNumRequire.style.display && phNumRequire.style.display !== 'none') {
        // If the requirements are already shown, hide them on click
        phNumRequire.style.display = 'none';
        phNumReveal.innerHTML = "See how your phone number should be formatted";
        
    } else {
        // If the requirements are already hidden, show them on click
        phNumRequire.style.display = 'block';
        phNumReveal.innerHTML = "Hide how your phone number should be formatted";
    } 
}


// On change of selection for the employment situation <select> element, show/hide the following element (automatically hidden)
function checkEmployment() {
    if (document.getElementById("other").selected) {
        // If "Other" is chosen, show the following element
        document.getElementById('clarifySituation').style.display = "block";
    } else {
        // If "Other" is not chosen, hide the following element
        document.getElementById('clarifySituation').style.display = "none";
    }
}

function resumeCheck() {
    if (document.getElementById("yes").selected) {
        document.getElementById("resumeOuter").style.display = "block";
    } else {
        document.getElementById("resumeOuter").style.display = "none";
        document.getElementById("fileInput").style.display = "none";
        document.getElementById("URLInput").style.display = "none";
    }
}

function resumeType() {
    if (document.getElementById("url").selected) {
        document.getElementById("URLInput").style.display = "block";
        document.getElementById("fileInput").style.display = "none";
    } else if...