Checking for relative vs absolute paths/URLs in PHP

Question

I need to implement functions to check whether paths and urls are relative, absolute, or invalid (invalid syntactically- not whether resource exists). What are the range of cases I should be looking for?

function check_path($dirOrFile) {
    // If it's an absolute path: (Anything that starts with a '/'?)
        return 'absolute';
    // If it's a relative path: 
        return 'relative';
    // If it's an invalid path:
        return 'invalid';
}

function check_url($url) {
    // If it's an absolute url: (Anything that starts with a 'http://' or 'https://'?)
        return 'absolute';
    // If it's a relative url:
        return 'relative';
    // If it's an invalid url:
        return 'invalid';
}

Answer 1

From Symfony FileSystem component to check if a path is absolute:

public function isAbsolutePath($file)
{
    return strspn($file, '/\\', 0, 1)
        || (strlen($file) > 3 && ctype_alpha($file[0])
            && substr($file, 1, 1) === ':'
            && strspn($file, '/\\', 2, 1)
        )
        || null !== parse_url($file, PHP_URL_SCHEME)
    ;
}