抽象类的属性可以在类外访问吗？

Is is possible to access a variable in a PHP abstract class, i.e.

abstract class Settings
{
    public $application = array
    (
        'Name' => 'MyApp',
        'Version' => '1.0.0',
        'Date' => 'June 1, 2017'
    )
}

echo Settings::application ['Name'];   // doesn't work

You could make the variable static, as long as it doesn't need to allow differentiation across instances (i.e. instance variable):

<?php
// example code

abstract class Settings
{
    public static $application = array
    (
        'Name' => 'MyApp',
        'Version' => '1.0.0',
        'Date' => 'June 1, 2017'
    );
}

echo Settings::$application ['Name'];

Run it in this playground example.

Though your original access of application was similar to that of a constant. use const to declare a Class constant:

abstract class Settings
{
    const application = array
    (
        'Name' => 'MyApp',
        'Version' => '1.0.0',
        'Date' => 'June 1, 2017'
    );
}

echo Settings::application ['Name'];

Run it in this playground example.

Abstract Classes can not be directly instantiated as they rely on child classes to fully implement the functionality.

So if you want to check your variable I would make new class and inherit from your Settings class. You will have to use it with inheritance anyway.

class MySettings extends Settings
{
  ....
}

$mySettings = new MySettings();
echo $mySettings->application['Name'];

More about abstract classes http://culttt.com/2014/03/26/abstract-classes/