path_resolve.php

<?php

if (!function_exists('path_resolve')) {
    /**
     * Converts parts of the file path to the correct full path.
     * @param string ...$parts
     * @return string|string[]|null
     */
    function path_resolve(string ...$parts): string
    {
        return preg_replace('#/+#', DIRECTORY_SEPARATOR,
            implode(DIRECTORY_SEPARATOR,
                array_reduce($parts, function ($path, $part) {
                    $part !== '' && $path[] = $part;
                    return $path;
                }, [])
            ));
    }
}

echo path_resolve('one', 'two');          // one/two
echo path_resolve('one/', 'two');         // one/two
echo path_resolve('one/', '///two');      // one/two
echo path_resolve('one/', 'two.json');    // one/two.json
echo path_resolve('one/', '/two/', 'th'); // one/two/th