OO PHP基本获取和设置方法“未定义变量”

Question

I've been reading up on OO PHP programming and encapsulation and I'm still finding it a little confusing.

I have this code:

class Item {

    private $id;
    private $description;

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

    public function getDescription() {
        return $this->$description;
    }

    public function setDescription($description) {
        $this->description = $description;
    }

}

In my testclass.php file, when I use the set and get Description functions like so:

$item = new Item(1234);
$item->setDescription("Test description");
echo $item->getDescription();

I get an error saying Undefined variable: description. Could someone please explain to me why this is, because I thought the point of a set method was to define the variable? I thought you declare the variable in the class, and then you define the variable when you use the set method so that it can be accessed with the get method?

Answer 1

Just want to add to the correct answer of @prisoner.

$this->description

is not the same as

$this->$description

because this is what you would call a "variable variable".

For example:

$this->description = "THIS IS A DESCRIPTION!"
$any_variable_name = "description";

echo $this->$any_variable_name; // will echo "THIS IS A DESCRIPTION"
echo $this->description // will echo "THIS IS A DESCRIPTION"
echo $this->$description // will result in an "undefined error" since $description is undefined.

Refer to http://php.net/manual/en/language.variables.variable.php for more details.

A useful example would be if you would like to access an a variable/function within a given structure.

Example:

$url = parseUrl() // returns 'user'

$this->user = 'jim';

$arr = array('jim' => 'the good man', 'bart' => 'the bad man');

echo $arr[$this->$url] // returns 'the good man'