yii2/docs/api/base/Object.md

A property is defined by a getter method (e.g. getLabel), and/or a setter method (e.g. setLabel). For example, the following getter and setter methods define a property named label:

private $_label;

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

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

Property names are case-insensitive.

A property can be accessed like a member variable of an object. Reading or writing a property will cause the invocation of the corresponding getter or setter method. For example,

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

If a property has only a getter method and has no setter method, it is considered as read-only. In this case, trying to modify the property value will cause an exception.

One can call hasProperty, canGetProperty and/or canSetProperty to check the existence of a property.