Added strict mode

01-front-controller.md

@@ -17,13 +17,15 @@ So instead of doing that, create a folder in your project folder called `public`
 Inside the `public` folder you can now create your `index.php`. Remember that you don't want to expose anything here, so put just the following code in there:
 
 ```php
-<?php 
+<?php declare(strict_types = 1); 
 
 require __DIR__ . '/../src/Bootstrap.php';
 ```
 
 `__DIR__` is a [magic constant](http://php.net/manual/en/language.constants.predefined.php) that contains the path of the directory. By using it, you can make sure that the `require` always uses the same relative path to the file it is used in. Otherwise, if you call the `index.php` from a different folder it will not find the file.
 
+`declare(strict_types = 1);` sets the current file to [strict typing](http://php.net/manual/en/functions.arguments.php#functions.arguments.type-declaration.strict). In this tutorial we are going to use this for all PHP files. This means that you can't just pass an integer as a parameter to a method that requires a string. If you don't use strict mode, it would be automatically casted to the required type. With strict mode, it will throw an Exception if it is the wrong type.
+
 The `Bootstrap.php` will be the file that wires your application together. We will get to it shortly.
 
 The rest of the public folder is reserved for your public asset files (like JavaScript files and stylesheets).
@@ -31,7 +33,7 @@ The rest of the public folder is reserved for your public asset files (like Java
 Now navigate inside your `src` folder and create a new `Bootstrap.php` file with the following content:
 
 ```php
-<?php 
+<?php declare(strict_types = 1);
 
 echo 'Hello World!';
 ```