build_autoloader()

autoload.php

<?php

namespace texdc\autoload;

use InvalidArgumentException;

/**
 * @param  array $aDirectoryMap
 * @return callable
 */
function build_autoloader(array $aDirectoryMap) : callable
{
    validate_directory_map($aDirectoryMap);
    return function(string $aClassName) use ($aDirectoryMap)
    {
        foreach ($aDirectoryMap as $namespace => $sourceDir) {
            if (stripos($aClassName, $namespace) !== false) {
                include_class_file($aClassName, $namespace, $sourceDir);
                break;
            }
        }
    };
}

/**
 * @param  array $aDirectoryMap
 * @throws InvalidArgumentException
 */
function validate_directory_map(array $aDirectoryMap)
{
    foreach ($aDirectoryMap as $sourceDir) {
        if (!is_dir($sourceDir) || !is_readable($sourceDir)) {
            throw new InvalidArgumentException("Invalid source directory [$sourceDir]");
        }
    }
}

/**
 * @param string $aClassName
 * @param string $aNamespace
 * @param string $aSourceDir
 */
function include_class_file(string $aClassName, string $aNamespace, string $aSourceDir)
{
    $fileName = str_replace([$aNamespace, '\\'], [$aSourceDir, DIRECTORY_SEPARATOR], $aClassName) . '.php';
    if (is_readable($fileName)) {
        include $fileName;
    }
}

function include_files(array $aFileList)
{
    foreach ($aFileList as $fileName) {
        $fileName = str_replace(['\\', '/'], DIRECTORY_SEPARATOR, $fileName);
        if (is_readable($fileName)) {
            include $fileName;
        }
    }
}

demo.php

<?php

require 'autoload.php';
use function texdc\autoload\{build_autoloader, include_files};

try {
    $ds = DIRECTORY_SEPARATOR;
    // could also be: $namespaceDirectoryMap = include 'autoload_namespaces.php';
    $namespaceDirectoryMap = [
        'foo\\bar\\' => __DIR__ . $ds . 'src' . $ds,
    ];
    spl_autoload_register(build_autoloader($namespaceDirectoryMap));
    
    // could also be: $includeFiles = include 'autoload_files.php';
    $includeFiles = [
        __DIR__ . $ds . 'src' . $ds . 'my_func.inc',
    ];
    include_files($includeFiles);
} catch (InvalidArgumentException $iae) {
    // die($iae->getMessage());
} finally {
    unset($namespaceDirectories, $includeFiles, $ds);
}

assert(class_exists('foo\bar\Test'));