yii2/docs/guide/concept-properties.md

Properties

In PHP, class member variables are also called properties. They are part of a class definition and are used to represent the state of a class instance. In practice, you may often want to do some special handling when a property is being read or modified. For example, you may want to trim a string when it is being assigned to a label property. You could use the following code to achieve this task:

$object->label = trim($label);

The drawback of the above code is that you have to call trim() everywhere whenever you modify the label property. And if in future, the label property has a new requirement, such as the first letter must be turned into upper case, you would have to modify all those places - a practice you want to avoid as much as possible.

To solve this problem, Yii introduces a base class called yii\base\Object to support defining properties based on getter and setter class methods. If a class needs such a support, it should extend from yii\base\Object or its child class.

Info: Nearly every core class in the Yii framework extends from yii\base\Object or its child class. This means whenever you see a getter or setter in a core class, you can use it like a property.

A getter method is a method whose name starts with the word get, while a setter method starts with set. The name after the get or set prefix defines the name of a property. For example, a getter getLabel() and/or a setter setLabel() defines a property named label, as shown in the following code:

namespace app\components;

use yii\base\Object;

class Foo extend Object
{
    private $_label;

    public function getLabel()
    {
        return $this->_label;
    }

    public function setLabel($value)
    {
        $this->_label = trim($value);
    }
}

Properties defined by getters/setters can be used like class member variables. The main difference is that when such a property is being read, the corresponding getter method will be called; and when the property is being assigned, the corresponding setter method will be called. For example,

// equivalent to $label = $object->getLabel();
$label = $object->label;

// equivalent to $object->setLabel('abc');
$object->label = 'abc';

A property defined by a getter without a setter is read only. Trying to assign a value to such a property will cause an yii\base\InvalidCallException. Similarly, a property defined by a setter without a getter is write only, and trying to read such a property will also cause an exception. It is not common to have write-only properties.

There are several special rules or limitations of the properties defined based on getters and setters.

• The names of such properties are case-insensitive. For example, $object->label and $object->Label are the same. This is because PHP method names are case-insensitive.
• If the name of such a property is the same as a class member variable, the latter will take precedence. For example, if the above Foo class has a member variable label, then the assignment $object->label = 'abc' will happen to the member variable instead of the setter setLabel().
• The properties do not support visibility. It makes no difference for the visibility of a property if the defining getter or setter method is public, protected or private.
• The properties can only be defined by non-static getters and/or setters. Static methods do not count.

Back to the problem we described at the very beginning, instead of calling trim() everywhere, we are calling it only within the setter setLabel(). And if a new requirement comes that the first letter of the label should be turned into upper case, we just need to modify the setLabel() method without touching any other code.