As of PHP 7.4.0, PHP can be configured to preload scripts into the opcache when the engine starts. Any functions, classes, interfaces, or traits (but not constants) in those files will then become globally available for all requests without needing to be explicitly included. That trades convenience and performance (because the code is always available) for baseline memory usage. It also requires restarting the PHP process to clear pre-loaded scripts, meaning this feature is only practical to use in production, not in a development environment.
Note that the optimal tradeoff between performance and memory may vary with the application. "Preload everything" may be the easiest strategy, but not necessarily the best strategy. Additionally, preloading is only useful when there is a persistent process from one request to another. That means while it can work in a CLI script if the opcache is enabled, it's generally pointless. The exception is when using preloading on FFI libraries.
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Preloading is not supported on Windows.
Configuring preloading involves two steps, and requires that the opcache be enabled. First, set the opcache.preload value in php.ini:
opcache.preload=preload.php
preload.php is an arbitrary file that will run once at server startup
(PHP-FPM, mod_php, etc.) and load code into persistent memory. In servers that startup as
root before switching to an unprivileged system user, or if PHP will be run as root
(not recommended), the opcache.preload_user
value can specify the system user to run the preloading. Running preloading as root is not allowed
by default. Set opcache.preload_user=root to explicitly allow it.
In the preload.php script, any file referenced by include,
include_once, require, require_once, or
opcache_compile_file() will be parsed into persistent memory. In the following example,
all .php files in the src directory will be preloaded, unless they
are a Test file.
<?php
$directory = new RecursiveDirectoryIterator(__DIR__ . '/src');
$fullTree = new RecursiveIteratorIterator($directory);
$phpFiles = new RegexIterator($fullTree, '/.+((?<!Test)+\.php$)/i', RecursiveRegexIterator::GET_MATCH);
foreach ($phpFiles as $key => $file) {
require_once $file[0];
}
?>Both include and opcache_compile_file() will work, but have different implications for how code gets handled.
A and b.php defines class
B that extends A, then opcache_compile_file() can
load those two files in any order. When using include, however, a.php
must be included first.There are caveats when enabling preloading, one of them being that it should be enabled via a php.ini file. Enabling it with a php-fpm pool configuration won't work, for instance, since preloading is global and not per-pool. To make sure that you successfully enabled preloading, you should check for a preload_statistics key in the output of opcache_get_status(). There should already be an opcache_statistics key, but that's something else entirely.PHP 8.1 comes with an inheritance cache that partially overlaps with what the preloading already does. If you enabled preloading on lower versions then migrated to PHP 8.1, you might want to turn off preloading and see if that comes with a performance penalty or not.