获取两个路径的相对路径

function getRealRelativePath($a, $b)
{

    $a_arr = explode('/', $a);
    $b_arr = explode('/', $b);
    $diff = $this->getDiff($a_arr, $b_arr);
    $path = str_repeat('../', count($diff));
    $b_diff = $this->getDiff($b_arr, $a_arr);
    $path .= implode('/', $b_diff);
    return $path;
}

function getDiff($a,$b){
    $i=0;
    while (isset($a[$i]) && isset($b[$i]) && $a[$i] == $b[$i]) {
        unset($a[$i]);
        $i++;
    }
    return $a;
}

$urla = "/home/web/lib/img/cache.php";
$urlb = "/home/web/api/img/temp/show.php";
$this->getRealRelativePath($urla,$urlb);
//b相对于a的路径为../../../api/img/temp/show.php
//不能使用array_diff_assoc和array_intersect_assoc等函数,
//因为下标和值一样的路径会存在,如上面的两个路径，会造成数据不正确