How Can I Properly Access External Objects Within a PHP Class?

Use global variables in the class

In PHP, global variables can be accessed from inside a class using the global keyword. However, this may cause code readability and maintainability issues. Consider the following code example:

$db = new DB_MySQL("localhost", "root", "", "test"); // 连接数据库
include_once("pagi.php");

$pagination = new pagi();
$records = $pagination->get_records("SELECT * FROM `table`");

in pagi.php file:

class pagi {

    public function get_records($q) {
        global $db;

        $x = $db->query($q);
        return $db->fetch($x);
    }

}

This code attempts to use a get_records variable outside the pagi class in a $db method. However, it results in a fatal error because global variables cannot be accessed via the -> operator.

Dependency Injection: A Better Way

A better way to access external objects is dependency injection. In this approach, the external object is passed as a parameter to the constructor or method of the class. For example:

$db = new DB_MySQL("localhost", "root", "", "test"); // 连接数据库
include_once("pagi.php");

$pagination = new Paginator($db);
$records = $pagination->get_records("SELECT the, fields, you, want, to retrieve FROM `table`");

class Paginator
{    
    protected $db;

    public function __construct(DB_MySQL $db)
    {
        $this->db = $db;
    }

    public function get_records($q) {
        $x = $this->db->query($q);
        return $this->db->fetch($x);
    }

}

In this case, the $db object is passed to the Paginator class through the constructor. This allows a class to perform queries using the $db object without setting it as a global variable or creating a new instance within the class.

Conclusion

While using global variables may be convenient, dependency injection is a more flexible and easier to maintain method of adding external functionality to a class. By passing dependencies explicitly, you decouple a class from a specific implementation and make it easier to test and maintain.

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