Problem detecting captcha creation

I'm logging in to the system, but I got to a part that I can not think of as a logic to run. When the user tries to log in and miss the password 3 times, a captcha will appear on the login screen, this I was able to do, I'm having trouble checking if this captcha was created when checking the login. I have the ready if, but I do not know how to implement it ...

code for login without verification:

if ($_POST) {
    $user = $_POST['txtu'];
    $senha = $_POST['txts'];
    session_start();

    if (efetuarLogin($conexao, $user, $senha)) {
        session_start();
        $_SESSION['nome'] = $user;
        $cpf = pegaCpfUsuarioLogado($conexao, $user);
        $_SESSION['tipoUsuario'] = pegaTipoUsuario($conexao, $cpf);
        mysqli_close($conexao);
        if ($_SESSION['tipoUsuario'] == 2) {
            $_SESSION['log'] = 'ativo';
            header("location: pag_pesquisarTec.php");
        } else {
            $_SESSION['log'] = 'ativoTecnico';
            header("location: pag_listar_chamados.php");
        }
    } else {
        // contabiliza os erros;
        if (empty($_SESSION['numErros'])) {
            $_SESSION['numErros'] = 1;
        } else {
            $_SESSION['numErros'] ++;
        }

        echo '<script> var r = confirm("Senha ou usuário incorreto!")
                                if (r === true) {
                                    window.location.assign("index.html");                   
                                } else {
                                    window.location.assign("index.html");
                                }
                                </script>';
    }
}

if to check the captcha:

    if ($_POST["palavra"] == $_SESSION["palavra"]) {

    } else {
        echo '<script>alert("Captcha errado!");'
        . 'window.location = "index.html"</script> ';
    }