计算 $b 相对于 $a 的路径的算法

relative_path.php

<?php

function rel($b, $a, $lvl = 1) {
    $first_b = retrieve_first($b);
    $first_a = retrieve_first($a);
    if (!$first_a) {
        return ltrim($b, '/');
    }
    if (!$first_b) {
        return '../' . rel($b, rm_first($a), $lvl);
    }
    if ($first_a == $first_b && $lvl) {
        return rel(rm_first($b), rm_first($a), $lvl);
    } else {
        return '../' . rel($b, rm_first($a), 0);
    }
}

function retrieve_first($str) {
    $lpos = strpos($str, '/');
    $rpos = strrpos($str, '/');
    if ($lpos == $rpos) {
        return '';
    } else {
        return substr($str, 1, strpos($str, '/', 1) - 1);
    }
}

function rm_first($str) {
    return substr($str, strpos($str, '/', 1));
}

$a = '/a/b/c/d/e.php';
$b = '/a/b/12/34/c/f.php';
$c = '/e.php';
//echo retrieve_first($a), PHP_EOL;
//echo retrieve_first($b), PHP_EOL;
//echo retrieve_first($c), PHP_EOL;
//echo rm_first($a), PHP_EOL;
//echo rm_first($b), PHP_EOL;
//echo rm_first($c), PHP_EOL;
echo rel($a, $b), PHP_EOL;
//echo rel($b, $c), PHP_EOL;
//echo rel($c, $a), PHP_EOL;