How to Properly Access External Variables (like a Database Object) within a PHP Class?

Using Global Variables in a Class

You are attempting to create a pagination class that utilizes an external variable. However, you are encountering an error: "Call to a member function query() on a non-object."

The issue arises because the external variable, $db, is not directly accessible within the class. To resolve this, we will explore two main approaches:

Approach 1: Dependency Injection

Dependency injection involves passing the database object as an argument to the class's constructor. This method ensures that the class has access to the necessary dependency.

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);
    }
}

Approach 2: Method Injection

Alternatively, you can pass the database object as an argument to the specific method that requires it. This approach is suitable when only a few methods need access to the dependency.

class Paginator
{
    public function get_records($q, DB_MySQL $db)
    {
        $x = $db->query($q);
        return $db->fetch($x);
    }
}

The choice between these approaches depends on your specific requirements. Dependency injection is preferred when multiple methods require the dependency, while method injection is suitable for limited dependency usage.

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