PHP-library installation via composer - composer-php

Question

Some PHP-libraries can be used after installing them via composer.
What this mean?
Is that only way to use those libraries or is there a way to use them copying the code in to correct location and referring them in the code?
Examples:
mPDF can be used (only?) via composer
https://mpdf.github.io/
PHPMailer can be used just copying files in correct location and referring to them
https://github.com/PHPMailer/PHPMailer

Answer 1

The thing with libraries is, that they might require other libraries and those libraries might require other libraries and so on. So downloading them and putting them in some location will be tedious and when two libraries require the same library in a different version you will run into problems. Composer will solve this issue for you by figuring out which libraries are needed to resolve all requirements and make sure to download a version that fits all or raise an error that the current collection of libraries contains incompatible libraries and which ones.
The other problem is finding the right location to store those libraries since PHP has to figure out where each class is stored you will either have to add require/include statements to your code and libraries which is tedious and will complicate future updates, e.g. when classes are renamed or removed. A way around this is is having a shared lib directory, but then you will run into problems when you have multiple projects requiring different library versions.
For libraries composer is the de facto standard and you will always need/want to install libraries in your project with it. It takes care of resolving correct versions, autoloading and updating making it incredibly helpful, especially if you were still around when composer was not a thing. I would use it even if I need no libraries just for the autoloading and the ability to later add libraries, when my project grows/changes.
edit: Even PHPMailer provides a composer.json even though it does not require other libraries, but if you install it via composer you can make sure that your system fulfills the requirements (PHP version & installed extensions) which you might miss otherwise, leading to a possibly long debugging session figuring out why some feature won't work.
edit You can use composer for projects on shared hosting as well. Instead of running the command on the server, you must then run it on your local machine or build server for the actual server. You can copy your project including the vendor folder to your shared host and things should work. The vendor folder contains all libraries and the autoload.php and can be copied along with your code.
In order to do that reliably, in your composer.json under config you can specify the platform you will run your code on. You should define the correct PHP version at the very least, but the installed extensions as well, to make sure you don't accidentally install libraries where you don't have the required extension. When you run composer install or composer update it will use these platform details as a basis to download libraries that match them. This is especially important when you have PHP 7 installed, but your host does not have it yet.
When running composer on a separate server than the one that uses code, a few options will not work like --apcu-autoloader, but you probably don't use them anyway.
If you run composer on your local machine and copy stuff over, you can improve the experience a bit by adding a few options to your composer install:
composer install --no-dev --prefer-dist --classmap-authoritative
You can get details about these options in the documentation. The important things are:
--no-dev, to possibly reduce the number of libraries downloaded to the ones needed for production (because we only want to run the project on our server not develop on it).
--prefer-dist (why is explained in the docs)
--classmap-authoritative or --optimize-autoloader, (the first might not work with some projects/libraries) but it will improve autoloading making your application a teeny, tiny bit faster in production
The first 2 options, you should not run if you copy your development environment, as they will not provide all dependencies for development. Maybe setup a second project that is just used for checking out the latest changes from git, running tests to make sure everything works, then removing vendor & running that command, (possibly make some changes to the config for prod) and finally copy things to your shared hosting environment. If you use something like gitlab that provides CI-capabilities, you could also do these steps on the ci-server and let that copy stuff, but it takes some time to set things up.