Using spl_autoloading within WordPress plugin

Question

I think the problem here is that you’re using a relative path in autoload.php inside the src/ subfolder,

Try for example to modify your src/autoload.php to use absolute paths instead:

\spl_autoload_register( function( $class )
{
    $dir = plugin_dir_path( __FILE__ );

    $file = str_replace( '\\', "https://wordpress.stackexchange.com/", $class ) . '.php';

    $path = $dir . $file;

    if( file_exists( $path ) )
        require_once $path;

} );

where we add a file_exists to be plugin class specific.

Update

Here’s the main /sample-plugin/sample-plugin.php file:

<?php
/**
 * Plugin Name: Sample Plugin
 */

namespace SamplePlugin;

require_once __DIR__ . '/src/autoload.php';
// require_once 'src/autoload.php';

$shortcode = new ShortCode();

Here’s the /simple-plugin/src/SamplePlugin/Shortcode.php file:

<?php
/**
 * Class Shortcode
 */

namespace SamplePlugin;

class Shortcode
{
    public function __construct()
    {
        print 'DEBUG: Greetings from the Shortcode instance!';
    }
}

You could also check out how Composer generates the vendor/autoload.php file, that’s included with:

require __DIR__ . '/vendor/autoload.php';