A few handy functions for dealing with paths in PHP in a cross-platform environment.

paths.php

<?php
if ( ! defined('IS_WIN')) {
    define('IS_WIN', (DIRECTORY_SEPARATOR === '\\'));
}

/**
 * Normalizes and joins path segments in accordance with the current OS standard.
 *
 * Usage:
 *
 *     join_path(__DIR__, 'foo', 'bar')
 *     join_path(array(__DIR__, 'bar'))
 * 
 * @param  string|array  $first  Either the first part of the path or an array of parts
 * @return string        The path.
 */
function join_path($first)
{
    // We support both arrays or func args.
    if (is_array($first)) {
        $base = array_shift($first);
        $parts = $first;
    } else {
        $base = $first;
        $parts = func_get_args();
        array_shift($parts);
    }

    // We only want to rtrim the base because if it is an abs path on *nix
    // it will have a leading slash.
    $base = rtrim($base, '\\/');
    $parts = array_map(function ($p) {
        return trim(str_replace(array('\\', '/'), DIRECTORY_SEPARATOR, $p), '\\/');
    }, $parts);

    // Put the base back onto the front
    array_unshift($parts, $base);

    return implode(DIRECTORY_SEPARATOR, $parts);
}

/**
 * Determines if the path given is an absolute path.
 * 
 * @param  string  $path  The path to check.
 * @return boolean        Whether it is an abs path.
 */
function is_abs($path)
{
    if (IS_WIN) {
        return ($path[1] === ':');
    }
    return ($path[0] === '/');
}