php如何计算文件的相对路径

<?php
/**
 * 求$b相对于$a的相对路径
 * @param string $a
 * @param string $b
 * @return string
 */
function getRelativePath ($a, $b)
{
  $patha = explode('/', $a);
  $pathb = explode('/', $b);
  $counta = count($patha) - 1;
  $countb = count($pathb) - 1;
  $path = "../";
  if ($countb > $counta) {
    while ($countb > $counta) {
      $path .= "../";
      $countb --;
    }
  }
  // 寻找第一个公共结点
  for ($i = $countb - 1; $i >= 0;) {
    if ($patha[$i] != $pathb[$i]) {
      $path .= "../";
      $i --;
    } else {
//判断是否为真正的第一个公共结点，防止出现子目录重名情况
      for ($j = $i - 1, $flag = 1; $j >= 0; $j --) {
        if ($patha[$j] == $pathb[$j]) {
          continue;
        } else {
          $flag = 0;
          break;
        }
      }
      if ($flag)
        break;
      else
        $i ++;
    }
  }
  for ($i += 1; $i <= $counta; $i ++) {
    $path .= $patha[$i] . "/";
  }
  return $path;
}
$a = "/a/c/d/e.php";
$b = "/a/c.php";
$path = getRelativePath($a, $b);
echo $path;