Check captcha does not work - php

Question

Help please deal with CAPTCHA. At me all is normal it is drawn, through Ajax the entered code is sent to file insert.php where at me the form handler. But the check does not work there. What do not enter in the input field, everything is considered correct
My form and ajax
<form action="/data/insert.php" method="POST" id="guest-form" name="reg">
<input id="username" type="text" name="name" placeholder="Username">
<input id="email" type="text" name="email" placeholder="E-mail">
<textarea id="message" type="text" name="text" placeholder="Your message"></textarea>
<img src="data/captcha.php" alt="Картинка" /><br />
<input id ="captcha" type="text" name="captcha" /><br />
<input id="submit" class="btn btn-default" type="button" value="Submit">
</form>
Ajax
$(document).ready(function() {
$('#submit').click(function() {
var username = $('#username').val();
var email = $('#email').val();
var message = $('#message').val();
var captcha = $('#captcha').val();
if(username || email || message || captcha === ''){
alert('Please input data in all fields');
}
else{
$.ajax({
type: "POST",
cache: false,
url: '/data/insert.php',
data: {username: username, email: email, message: message, captcha: captcha},
success: function(data) {
}
});
}
});
});
The php handler
require_once 'db.php';
require_once 'captcha.php';
session_start();
if (isset($_SESSION["captcha"]) && $_SESSION["captcha"]===$_POST["captcha"]){ echo "Текст введен верно"; }
else {
echo "Текст введен не верно";
}
unset($_SESSION["captcha"]);
if (isset($_POST['username']) && isset($_POST['email']) && isset($_POST['message'])){
$username = $_POST['username'];
$email = $_POST['email'];
$message = $_POST['message'];
$db_host = "localhost";
$db_user = "alekspvn"; // Логин БД
$db_password = "123"; // Пароль БД
$db_table = "book"; // Имя Таблицы БД
$connect_db=mysql_connect(HOST, MYSQL_USER, MYSQL_PASS)
or die("No connection with SQL");
mysql_select_db("guests_db",$connect_db);
mysql_query("SET NAMES 'utf8'",$connect_db);
$result = mysql_query ("INSERT INTO ".$db_table." (username,email,message) VALUES ('$username','$email','$message')");
if ($result = 'true'){
echo "Информация занесена в базу данных";
}else{
echo "Информация не занесена в базу данных";
}
}
?>