Composer/PSR - How to autoload functions?

How can I autoload helper functions (outside of any class)? Can I specify in composer.json some kind of bootstrap file that should be loaded first?

You can autoload specific files by editing your composer.json file like this:

"autoload": {
    "files": ["src/helpers.php"]
}

(thanks Kint)

After some tests, I have came to the conclusions that adding a namespace to a file that contains functions, and setting up composer to autoload this file seems to not load this function across all the files that require the autoload path.

To synthesize, this will autoload your function everywhere:

composer.json

"autoload": {
    "files": [
        "src/greetings.php"
    ]
}

src/greetings.php

<?php
    if( ! function_exists('greetings') ) {
        function greetings(string $firstname): string {
            return "Howdy $firstname!";
        }
    }
?>

...

But this will not load your function in every require of autoload:

composer.json

"autoload": {
    "files": [
        "src/greetings.php"
    ]
}

src/greetings.php

<?php
    namespace You;

    if( ! function_exists('greetings') ) {
        function greetings(string $firstname): string {
            return "Howdy $firstname!";
        }
    }
?>

And you would call your function using use function ...; like following:

example/example-1.php

<?php
    require( __DIR__ . '/../vendor/autoload.php' );

    use function You\greetings;

    greetings('Mark'); // "Howdy Mark!"
?>

1. Add autoload info in composer.json

{
    "autoload": {
        "psr-4": {
            "Vendor\\Namespace\\": "src/"
        }
    }
}

2. Create a OwnFunctions.php with your functions in src/Functions folder

// recommend
// http://php.net/manual/en/control-structures.declare.php
declare(strict_types=1);

namespace Vendor\Namespace\Functions\OwnFunctions;

function magic(int $number): string {
    return strval($number);
}

3. In your index.php require composer autoload

declare(strict_types=1);
require_once __DIR__ . '/vendor/autoload.php';

use function Vendor\Namespace\Functions\OwnFunctions\magic;

echo magic(1);

// or you can use only OwnFunctions namespace
use Vendor\Namespace\Functions\OwnFunctions;

echo OwnFunctions\magic(1);

This also can be done with const

use const Vendor\Namespace\Functions\OwnFunctions\someConst;
echo someConst;

Official docs