<form class='sign_up' action='' method='POST'>
            <input id='email' type="text" name="email" placeholder="Email">
            <input id='username' type="text" name="username" placeholder="Username">
            <input type="password" name="password" placeholder="Password">
            <input id='registerBtn' name='registerBtn' type="submit" value="Create Account">
        </form>

$(".sign_up").validate({
        errorElement: 'div',
        rules: {
            email: {
                required: true,
                maxlength: 60,
                email: true,
                remote: {
                    url: 'usernameChecker.php',
                    type: 'post',
                    data: {
                        email: function(){
                            return $("#email").val();
                        },
                    },
                },
            },

            username: {
                required: true,
                maxlength: 15,
                alphanumeric: true,
                remote: {
                    url: 'usernameChecker.php',
                    type: 'post',
                    data: {
                        username: function(){
                            return $("#username").val();
                        },
                    },
                },
            },

            password: {
                required: true,
                minlength: 7,
            }

        },

        messages: {
            email: {
                required: "*Please enter your email.",
                maxlength: "*Emails should not exceed 60 characters.",
                email: "*This is not a valid email.",
                remote: "*That email is already taken."
            },

            username: {
                required: "*Please enter a username.",
                maxlength: "*Username should not exceed 15 characters.",
                alphanumeric: "*Usernames should contain only letters, numbers or underscores.",
                remote: "*That username is already taken."
            },

            password: {
                required: "*Please enter a password.",
                minlength: "*Your password should be at least 7 characters long.",
            }
        },

          submitHandler: function(form) {
          $("#registerBtn").prop("disabled", "disabled");
            $(form).ajaxSubmit();
          }
    });