PHP Programming/Caching

A cache is a collection of duplicate data, where the original data is expensive to fetch or compute (usually in terms of access time) relative to the cache. In PHP, caching is used to minimize page generation time.

Classification

PHP provides several cache systems:

Name	Data stored	Flush
Instance cache	PHP object. Ex: if (null === $x) { $x = 1; }	Restart the script (ex: refresh the Web page).
Session cache	PHP object	Empty the navigator cookies.
OPcache	Opcode	opcache_reset();
APCu	RAM users variables	apc_clear_cache();
Navigator cache	Rendering	CTRL + F5
ESI	Web page parts	Depends on the CDN or proxy server used.
Framework cache	Configurations, translations	Example of Symfony: php bin/console cache:clear empty the temporary var/cache files.
Proxy	Entire web pages	For examples, see: Varnish, HAProxy.
NoSQL database	Key-value pairs	For examples, see: Memcached, Redis.
ORM cache	Annotations, SQL requests or their results	Example with Doctrine: php bin/console doctrine:cache:clear-metadata php bin/console doctrine:cache:clear-query php bin/console doctrine:cache:clear-result or: bin/console cache:pool:clear doctrine.query_cache_pool doctrine.result_cache_pool doctrine.system_cache_pool
Chain cache	Everything	Use each included cache flushes.

PHP basically has two main types of caching: 'output caching' and 'parser caching'. PHP 'output caching' saves a chunk of data somewhere that can later be read by another script faster than it can generate it. 'Parser caching' is specific feature. PHP is a scripting language and code is not compiled or optimized to a particular computer. Every PHP file must be parsed and that takes time. This type of time minimization is 'parser caching'.

Parser caching

Include caching

Example:

File:class.test.php

<?php
class test
{
    function hello()
    {
        echo "Hello world!\n";
    }
}
echo "Class loaded\n";

File:program.php

<?php
require_once("class.test.php");
$obj1 = new test;
$obj1->hello();
require_once("class.test.php");
$obj2 = new test;
$obj2->hello();

output:

Class loaded
Hello world!
Hello world!

Array caching

Example:

File:program.php

<?php
global $sum_cache; 
$sum_cache=array();
function sum($nr)
{
    global $sum_cache;
    
    if (isset($sum_cache[$nr])) {
       echo "sum(".$nr.")=" . $sum_cache[$nr] . " from cache\n";
       return $sum_cache[$nr];
    }

    if ($nr <= 0) {
       $sum_cache[$nr] = 0;
    } else {
       $sum_cache[$nr] = sum($nr - 1) + $nr;
    }

    echo "sum(".$nr.")=" . $sum_cache[$nr] . " computed\n";
    return $sum_cache[$nr];
}
sum(3);
sum(4);

output:

sum(0)=0 computed
sum(1)=1 computed
sum(2)=3 computed
sum(3)=6 computed
sum(3)=6 from cache
sum(4)=10 computed

Session caching

Example:

file:program.php

<?php
session_start();
function find_my_name()
{
    //perhaps some time-consuming database queries
    return "Bill"; 
}
if (isset($_SESSION["hello"])) {
    echo "cached\n";
    session_destroy();
} else {
    echo "computed\n";
    $_SESSION["hello"] = "My Name is " . find_my_name() . ".\n";
}
echo $_SESSION["hello"];

output:

computed
My Name is Bill.

output after refresh:

cached
My Name is Bill.

Shared variables

Example:

file:program.php

<?php
class test
{
    var $list = array();

    function load_list()
    {
        // some less time consuming database queries
        $this->list[0]["info"] = "small info nr 1";
        $this->list[1]["info"] = "small info nr 2";
        $this->list[2]["info"] = "small info nr 3";
    }

    function load_element_detail(&$data)
    {
        // some very time consuming database queries
        $data["big"] = "added big info, maybe structure of objects";
    }

    function get_element($nr)
    {
        return $this->list[$nr];
    }

    function print_element($nr)
    {
        echo "this->list[${nr}]['info'] = '" . $this->list[$nr]['info'] . "'\n";
        echo "this->list[${nr}]['big']  = '" . $this->list[$nr]['big'] . "'\n";
    }
}

$obj = new test;

$obj->load_list();
$obj->print_element(0);

$element = &$obj->get_element(0);

$obj->load_element_detail($element);
$obj->print_element(0);

output:

$this->list[0]["info"]="small info nr 1"
$this->list[0]["big"]=""
$this->list[0]["info"]="small info nr 1"
$this->list[0]["big"]="added big info, maybe structure of objects"

Output Caching

The server cache policy is included into HTTP header, visible with cURL (with option "I" for header):

curl -I http://example.org

Example:

curl -I https://en.wikibooks.org/
HTTP/2 301 
date: Sun, 02 Jan 2022 11:50:58 GMT
server: mw1429.eqiad.wmnet
x-content-type-options: nosniff
p3p: CP="See https://en.wikibooks.org/wiki/Special:CentralAutoLogin/P3P for more info."
vary: Accept-Encoding,X-Forwarded-Proto,Cookie,Authorization
cache-control: s-maxage=1200, must-revalidate, max-age=0
last-modified: Sun, 02 Jan 2022 11:50:58 GMT
location: https://en.wikibooks.org/wiki/Main_Page
content-length: 0
content-type: text/html; charset=utf-8
age: 1139
x-cache: cp3062 miss, cp3060 hit/4
x-cache-status: hit-front
server-timing: cache;desc="hit-front", host;desc="cp3060"
strict-transport-security: max-age=106384710; includeSubDomains; preload
report-to: { "group": "wm_nel", "max_age": 86400, "endpoints": [{ "url": "https://intake-logging.wikimedia.org/v1/events?stream=w3c.reportingapi.network_error&schema_uri=/w3c/reportingapi/network_error/1.0.0" }] }
nel: { "report_to": "wm_nel", "max_age": 86400, "failure_fraction": 0.05, "success_fraction": 0.0}
permissions-policy: interest-cohort=()
set-cookie: WMF-Last-Access=02-Jan-2022;Path=/;HttpOnly;secure;Expires=Thu, 03 Feb 2022 12:00:00 GMT
set-cookie: WMF-Last-Access-Global=02-Jan-2022;Path=/;Domain=.wikibooks.org;HttpOnly;secure;Expires=Thu, 03 Feb 2022 12:00:00 GMT