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PHP 7.4 preload

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There are two last small - but cool - features I want to talk about.

Huh? Preload?

For the first, search for "Symfony preload" to find a blog post talking about it: "New in Symfony 4.4: Preloading Symfony Applications in PHP 7.4".

Here's the deal: in PHP 7.4 a new feature was added called "preloading". Basically, in your php.ini file, you can point an opcache.preload setting at a file that contains a list of all the PHP files that your application uses.


You may also need to set an opcache.preload_user setting set to your web server user (e.g. www-data).

By doing this, when PHP starts, it will "preload" those files into OPcache. You're effectively giving your web-server a "head" start: telling it to load the source code it will need into memory now so that it's ready when you start serving traffic.

What's the catch? Well, first, you need to create this "list of files", which we'll talk about in a minute. Second, each time these files change - so on each deploy - you need to restart your web server. And third, until PHP 7.4.2, this feature was a little buggy. It should be fine now, but there still could be some bugs left. Proceed with caution.

The Generated Preload File

So how does Symfony fit into this? Symfony knows a lot about your app, like which classes your app uses. And so, it can build that "preload" file automatically.

Check it out, at your terminal, clear the prod cache:

php bin/console cache:clear --env=prod

Now, in PhpStorm, check out the var/cache/prod/ directory... here it is: App_KernelProdContainer.preload.php. This file - which basically includes a bunch of classes - is a PHP 7.4 preload file. All you need to do is update the opcache.preload setting in php.ini to point to this file, restart your web server any time you deploy and, voilĂ ! Instant performance boost!

How much of a boost? I'm not sure. It's such a new feature that benchmarks are only starting to be released. The blog post says 30 to 50%, I've seen other places saying more like 10 or 15%. Either way, if you can get your system set up to use it, free performance!

Next, let's talk about one last feature: a command you can run to make sure all your service wiring and type-hints are playing together nicely. Because in our app, there is a problem.