类变量 - 何时使用它们

Question

How does one know when to put variables into the class rather than inside the class functions? For example - This database class is instantiated by its sub-class and it also instantiates its sub-class. It has no class variables.

class database extends one_db      
{
    function __construct()  
    {
        one_db::get();
    }

    public function pdo_query()
    {
    }

    public function query($query) 
    {
        return one_db::$db->query($query);
    }

    private function ref_arr(&$arr)  // pdo_query will need this later. 
    { 
        $refs = array(); 
        foreach($arr as $key => $value) 
        {  
            $refs[$key] = &$arr[$key];
        } 
        return $refs;
    }   
}

Howeve I could just as well pull out the $query variabe like this

class database extends one_db      
{
    protected $query;

    function __construct()  
    {
        one_db::get();
    }

    public function pdo_query()
    {
    }

    public function query($query) 
    {
        $this->query=$query
        return one_db::$db->query($this->query);
    }

    private function ref_arr(&$arr)  // pdo_query will need this later. 
    { 
        $refs = array(); 
        foreach($arr as $key => $value) 
        {  
            $refs[$key] = &$arr[$key];
        } 
        return $refs;
    }   
}

I would assume that this only needs to be done when the variable is shared between multiple class functions but I'm not too sure.

Answer 1

There are 3 types on variables to be used in a class/object:

1. An instance variable - to be used as an object-wide variable, which all methods should have access to. Saving a databse connection is a good idea for an instance variable.
2. A static variable - to be used when there is no need for an object instance. A static counter of some some sort is usually a static variable.
3. A method variable - which is only contained within its function. Internal methodical variables should go in this category.

Your choice depending on your needs.