What is the difference between PHP class declaration and instantiation?

Question

I am a newbie. I found that many tool classes can be used by directly declaring them. For example, {code...} in laravel makes me confused. Does declaring a class variable instantiate the code? After searching on Baidu, no one asked about this issue in PHP language, only Java, and Java is like this...

PHP中文网 · Answer

PHP doesn’t have this kind of syntax
What you see should be something like this
Route::get('/', function (IlluminateHttpRequest $request) {

return view('welcome', ['a'=>$request->get('a',1)]);

});
This is dependency injection
http://www.golaravel.com/lara...

过去多啦不再A梦 · Answer

The following is wrong: After carefully looking at the source code, is the answer like this?
at

Request $request;
$request->get();

Before these two pieces of code, I used use IlluminateHttpRequest;

use just uses the namespace, but if you want to call the class, you must load the class file (require), or load it automatically. , lavavel uses automatic loading, so in our opinion, using IlluminateHttpRequest is equivalent to being able to use the Request class directly. In fact, it is not. Laravel actually does a lot of work when using the Request class!
Laravel’s automatic loading function is as follows:

 public static function getLoader()
    {
        if (null !== self::$loader) {
            return self::$loader;
        }

        spl_autoload_register(array('ComposerAutoloaderInit67db7509c61e60a4f92e012c704d3566', 'loadClassLoader'), true, true);
        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
        spl_autoload_unregister(array('ComposerAutoloaderInit67db7509c61e60a4f92e012c704d3566', 'loadClassLoader'));

        $map = require __DIR__ . '/autoload_namespaces.php';
        foreach ($map as $namespace => $path) {
            $loader->set($namespace, $path);
        }

        $map = require __DIR__ . '/autoload_psr4.php';
        foreach ($map as $namespace => $path) {
            $loader->setPsr4($namespace, $path);
        }

        $classMap = require __DIR__ . '/autoload_classmap.php';
        if ($classMap) {
            $loader->addClassMap($classMap);
        }

        $loader->register(true);

        $includeFiles = require __DIR__ . '/autoload_files.php';
        foreach ($includeFiles as $fileIdentifier => $file) {
            composerRequire67db7509c61e60a4f92e012c704d3566($fileIdentifier, $file);
        }

        return $loader;
    }
}

It can be seen that if you use an uninstantiated class, it will be automatically loaded and instantiated.