Demo of PHP Preload

myclass.php

<?php
echo "myclass.php required".PHP_EOL;
class MyClass
{
        public function MyFunction():void
        {
                echo "This is a function!".PHP_EOL;
        }
}

myfile.php

<?php
echo "Start of myfile.php".PHP_EOL;
echo "Opcache is ".(opcache_get_status()?"on, files will be cached":"off, files will not be cached").PHP_EOL;
spl_autoload_register(function(){
        echo "Calling autoloader".PHP_EOL;
        require_once "myclass.php";
});
$thing=new MyClass();
$thing->MyFunction();

output

nothing:
Start of myfile.php
Opcache is off, files will not be cached
Calling autoloader
myclass.php required
This is a function!
opcache:
Start of myfile.php
Opcache is on, files will be cached
Calling autoloader
myclass.php required
This is a function!
preload:
myclass.php required
Start of myfile.php
Opcache is on, files will be cached
This is a function!

php-nothing.ini

[opcache]
; Still need to enable extension so we don't get error
zend_extension=opcache.so
opcache.enable=0
opcache.enable_cli=0

php-opcache.ini

[opcache]
opcache.enable=1
zend_extension=opcache.so
opcache.enable_cli=1

php-preload.ini

[opcache]
opcache.enable=1
zend_extension=opcache.so
opcache.enable_cli=1
opcache.preload=myclass.php

run.php

<?php
$configs=["nothing","opcache","preload"];
foreach($configs as $config)
{
        echo "$config:".PHP_EOL;
        passthru("php -c php-$config.ini myfile.php");
}

Run with php run.php

In summary - php runs the file specified by opcache.preload on startup (typically server startup, but in this case CLI startup), takes all symbols (functions/classes) defined by it and make them accessible to any invocation of PHP (typically webpage load).

In this example, with no opcache loaded, the loading precedes as normal - when the class is constructed, PHP calls the autoloader, it loads the file from disk, compiles the file, and runs it to get the class. With opcache loaded, this procedure remains the same, but the file may be loaded from memory instead of disk if it has already been loaded. With preload, however, the class is loaded when PHP starts, so the autoloader is never called. (Note that typically you probably wouldn't want to specify the preload file to point directly to a class, but to some sort of autoloader for your program that will include all the classes you want).