如何在PHP类中维护静态成员状态？

Question

In Java, a static member maintains its value for all instances of the class. Can this be done in PHP? I remember hitting this issue a few years ago and my current test confirms that static member does not maintain its state. So I guess, in PHP a class is unloaded and all its state destroyed after each request.

index.php

include('cache.php');

$entityId=date('s');
$uri='page'.$entityId;

$cache = new Cache();
$cache->cacheUrl($uri, $entityId);

cache.php

class Cache {
    private static $URL_CACHE;

    public function cacheUrl($url, $entityId) {
        echo '<br>caching '.$url.' as '.$entityId;
        $URL_CACHE[$url]=$entityId;

        echo '<br>Cache content:<br>';
        foreach ($URL_CACHE as $key => $value) {
            echo 'Key: '.$key.' Value: '.$value.'<br>';
        }
    }

}

Output (each time I get a single Key=>Value)

caching test33 as 33
Cache content:
Key: test33 Value: 33

I understand we do not have the concept of a JVM in PHP. Is there still a way to do this in a standard installation of PHP (typical VPS hosting service with a cPanel)?

Answer 1

During script execution all instances of class have access to static variable and can change it.

Here's a test (note self:: when acessing $URL_CACHE):

class Cache {
    private static $URL_CACHE;

    public function cacheUrl($url, $entityId) {
        echo '<br>caching '.$url.' as '.$entityId . '<br />';
        self::$URL_CACHE[$url]=$entityId;

        echo '<br>Cache content:<br>';
        foreach (self::$URL_CACHE as $key => $value) {
            echo 'Key: '.$key.' Value: '.$value.'<br />';
        }
    }

}


$cache = new Cache();
$cache->cacheUrl('uri1', 'ent1');

$ya_cache = new Cache();
$ya_cache->cacheUrl('uri2', 'ent2');

Output is similar to:

<br>caching uri1 as ent1<br />
<br>Cache content:<br>Key: uri1 Value: ent1<br />

<br>caching uri2 as ent2<br />
<br>Cache content:<br>Key: uri1 Value: ent1
<br />Key: uri2 Value: ent2<br />

Code on eval: https://3v4l.org/WF4QA

But if you want to store self::$URLS_CACHE between script executions - use storages like databases, files, key-value storages, whatever.