script.js

$('document').ready(function() {
    var username_state = false;
    var email_state = false;
    $('#username').on('blur', function() {
        var username = $('#username').val();
        if (username == '') {
            username_state = false;
            return;
        }
        $.ajax({
            url: 'signup.php',
            type: 'post',
            data: {
                'username_check': 1,
                'username': username,
            },
            success: function(response) {
                if (response == 'taken') {
                    username_state = false;
                    $('#username').parent().removeClass();
                    $('#username').parent().addClass("form_error");
                    $('#username').siblings("span").text('Username already in use');
                } else if (response == 'not_taken') {
                    username_state = true;
                    $('#username').parent().removeClass();
                    $('#username').parent().addClass("form_success");
                    $('#username').siblings("span").text('Username available');
                }
            }
        });
    });
    $('#email').on('blur', function() {
        var email = $('#email').val();
        if (email == '') {
            email_state = false;
            return;
        }
        $.ajax({
            url: 'signup.php',
            type: 'post',
            data: {
                'email_check': 1,
                'email': email,
            },
            success: function(response) {
                if (response == 'taken') {
                    email_state = false;
                    $('#email').parent().removeClass();
                    $('#email').parent().addClass("form_error");
                    $('#email').siblings("span").text('Email already in use');
                } else if (response == 'not_taken') {
                    email_state = true;
                    $('#email').parent().removeClass();
                    $('#email').parent().addClass("form_success");
                    $('#email').siblings("span").text('Email available');
                }
            }
        });
    });
});