Laravel 5.6 Call to undefined function App \ Http \ Controllers \ is_countable ()

I am new to laravel and I do not have much knowledge in php I want you to help me please to identify the error !!

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\DB;

use App\User;

class UserController extends Controller
{
public function register(Request $request) {
    // Recoger post
    $json = $request -> input('json', null);
    $params = json_decode($json);

    $email = (!is_null($json) && isset($params -> email)) ? $params -> email : null;
    $name = (!is_null($json) && isset($params -> name)) ? $params -> name : null;
    $surname = (!is_null($json) && isset($params -> surname)) ? $params -> surname : null;
    $role = 'ROLE_USER';
    $password = (!is_null($json) && isset($params -> password)) ? $params -> password : null;

    if (!is_null($email) && !is_null($password) && !is_null($name)) {
        // Crear el usuario
        $user = new User();

        $user -> email = $email;
        $user -> name = $name;
        $user -> surname = $surname;
        $user -> role = $role;

        // Encriptar contraseña
        $pwd = hash('sha256', $password);
        $user -> password = $pwd;

        // Comprobar usuarios duplicados
        $isset_user = User::where('email', '=', $email) -> first();

        if (count($isset_user) === 0) {
            // Guardar el usuario
            $user -> save();

            $data = array(
                'status' => 'success',
                'code' => 200,
                'message' => 'Usuario registrado correctamente'
            );
        } else {
            // No guardar porque ya existe
            $data = array(
                'status' => 'error',
                'code' => 400,
                'message' => 'Usuario duplicado, no puede registrarse'
            );
        }
    } else {
        $data = array(
            'status' => 'error',
            'code' => 400,
            'message' => 'Usuario no creado'
        );
    }

    return response() -> json($data, 200);
}

public function login(Request $request) {
    echo 'Acción login';
    die();
}

}

This sent by post on a json using postman

{"name": "Juan Pablo", "surname": "Pachar Viñan", "email": "[email protected]", "password": "jppachar"}

and I get the following error:

"Call to undefined function App\Http\Controllers\is_countable()"

In this line of code I detect the error

if (count($isset_user) === 0) {

I'm using PHP 7.2