In our code, we define a list of books. So far, we have only three books, but you can guess that if we want to make this application useful, the list will grow way more. Storing the information inside your code is not practical at all, so we have to start thinking about externalizing it.

If we think in terms of separating the code from the data, there is no need to keep using PHP arrays to define the books. Using a less language-restrictive system will allow people who do not know PHP to edit the content of the file. There are many solutions for this, like CSV or XML files, but nowadays, one of the most used systems to represent data in web applications is JSON. PHP allows you to convert arrays to JSON and vice versa using just a couple of functions: json_encode and json_decode. Easy, right?

Save the following into books.json:

[
    {
        "title": "To Kill A Mockingbird",
        "author": "Harper Lee",
        "available": true,
        "pages": 336,
        "isbn": 9780061120084
    },
    {
        "title": "1984",
        "author": "George Orwell",
        "available": true,
        "pages": 267,
        "isbn": 9780547249643
    },
    {
        "title": "One Hundred Years Of Solitude",
        "author": "Gabriel Garcia Marquez",
        "available": false,
        "pages": 457,
        "isbn": 9785267006323
    }
]

The preceding code snippet is a JSON representation of our array in PHP. Now, let's read this information with the function file_get_contents, and transform it to a PHP array with json_decode. Replace the array with these two lines:

$booksJson = file_get_contents('books.json');
$books = json_decode($booksJson, true);

With just one function, we are able to store all the content from the JSON file in a variable as a string. With the function, we transform this JSON string into an array. The second argument in json_decode tells PHP to transform it to an array, otherwise it would use objects, which we have not covered as yet.

When referencing files within PHP functions, you need to know whether to use absolute or relative paths. When using relative paths, PHP will try to find the file inside the same directory where the PHP script is. If not found, PHP will try to find it in other directories defined in the include_path directive, but that is something you would like to avoid. Instead, you could use absolute paths, which is a way to make sure the reference will not be misunderstood. Let's see two examples:

$booksJson = file_get_contents('/home/user/bookstore/books.json');
$booksJson = file_get_contents(__DIR__, '/books.json');

The constant __DIR__ contains the directory name of the current PHP file, and if we prefix it to the name of our file, we will have an absolute path. In fact, even though you might think that writing down the whole path by yourself is better, using __DIR__ allows you to move your application anywhere else without needing to change anything in the code, as its content will always match the directory of the script, whereas the hardcoded path from the first example will not be valid anymore.