get a relative path from another path

getrelativepath.php

<?php

header('Content-Type: text/html; charset=utf-8');

function getRelativePath($path, $reference)
{
    //normalisation des parametres
    $path = (string) $path;
    if ('/' == $path[0]) {
        $path = substr($path, 1);
    }

    $reference = (string) $reference;
    if ('/' == $reference[0]) {
        $reference = substr($reference, 1);
    }

    $reference = explode('/', $reference);
    $path = explode('/', $path);
    array_map('urldecode', $reference);
    array_map('urldecode', $path);

    //si les 2 path sont identiques on retourne une string vide
    if ($path == $reference) {
        return '';
    }

    $filename = array_pop($path); //on ne touche pas au filename (ie le dernier segment du path)

    // on determine l'index du dernier segment consecutif en fonction du path de reference
    $index = 0;
    foreach ($reference as $offset => $value) {
        if (! isset($path[$offset]) || $value != $path[$offset]) {
            break;
        }
        $index++;
    }
    //on deduit le nombre de segment identique en fonction du path de reference
    $start_index = count($reference) - $index;

    $res = array_merge(
        array_fill(0, $start_index, '..'),
        array_slice($path, $index),
        array($filename)
    );
    array_map('rawurlencode', $res);

    return implode('/', $res);
}

/**
 *  Example Code Usage
 */

$tests = array(
    array(
        'path' => '/path/to/my/file.php',
        'ref'  => '/path/to/parent.php',
        'test' => 'Reference path is a parent directory',
    ),
    array(
        'path' => '/path/to/my/file.php',
        'ref'  => '/path/to/my/other_file.php',
        'test' => 'reference path in the same directory'
    ),
    array(
        'path' => '/path/to/my/file.php',
        'ref'  => '/path/to/my/other/file.php',
        'test' => 'reference path in a child directory'
    ),
    array(
        'path' => '/path/to/my/file.php',
        'ref'  => '/path/to/my/file.php',
        'test' => 'reference path is the same'
    ),
);

echo '<pre>', PHP_EOL;
foreach ($tests as $args) {
    $res = getRelativePath($args['path'], $args['ref']);
    echo $args['test'], PHP_EOL,
        "PATH: ", $args['path'], PHP_EOL,
        "REFERENCE: ", $args['ref'], PHP_EOL,
        "RELATIVE PATH: ", var_export($res, true), PHP_EOL,
        "---", PHP_EOL, PHP_EOL;
}