A small study on the relative path method

As in the title, find the relative paths of two paths, and ask for a brick!

/**
* Find the relative path of two paths
* @param string $patha path a
* @param string $pathb path b
* @author Joychao
* @link http://www.joychao .cc
* @return string relative path
*/
function getRelativePath($patha,$pathb){
 $arr_a=explode('/',trim(dirname($patha),'/'));
 $arr_b =explode('/',trim(dirname($pathb),'/'));
 $n=min(count($arr_a),count($arr_b));//Use the shortest path to loop
 $flag =true;//Mark bit [whether the mark has no intersection at all]
 for($i=0;$i<$n;$i++){
 if($arr_a[$i]==$arr_b[$i]) {
 unset($arr_a[$i],$arr_b[$i]);//Remove the same part before
 }else{
 if($i==0)
 $flag=false;//There are no two paths Intersection
 break;//Stop the loop
 }
 }

 $str=$flag?str_repeat('../',count($arr_b)+1):'/';//If there is no intersection, it is the root directory" /"[Linux situation, Windows can change it by itself]
 return $str.join('/',$arr_a);//Splice and return
}
//TEST

$a ='/a/b/c/d /e.php';
$b ='/a/b/12/34/c.php';
echo 'Path a:'.$a;
echo 'Path b:'. $b;
echo 'The relative path between path a and path b (use c.php to access the e.php path) is:';
echo getRelativePath($a,$b);
// -----OUTPUT-------------=
Path a:/a/b/c/d/e.php
Path b:/a/b/12/34/c .php
The relative path between path a and path b (use c.php to access the e.php path) is: ../../../c/d