<?php
//计算出两个文件的相对路径即path2相对于$path1的相对路径
// http://www.manongjc.com/article/1342.html
function get_relative_path($path1,$path2){
    $arr1 = explode(DIRECTORY_SEPARATOR,dirname($path1));
    $arr2 = explode(DIRECTORY_SEPARATOR,dirname($path2));
    $length = count($arr2);
    for($i = 0; $i < $length; $i++){
        if($arr1[$i] != $arr2[$i]){ //进行路径匹配
            break;
        }
    }
    if($i == 0){                    //根目录不同
        return '不在同一个目录中';
    }else if($i != 0 && $i < $length){        //在同一个个目录下
        $separator = array_fill(0, ($length-$i), '..');
        return(implode(DIRECTORY_SEPARATOR, array_merge($separator, array_slice($arr1, $i))));
    }else if($i == $length){                //路径完全相同 http://www.manongjc.com/article/1343.html
        return '.';
    }
}
$path1 = 'F:\Apache\www\temp\php_demo\demo.md'; //参照路径path1
$path2 = 'F:\Apache\www\temp\php_demo\demo.md';
$path3 = 'F:\Apache\www\temp\sd\ss.d\sdsfd';
$path4 = 'G:\FFOutput';
var_dump(get_relative_path($path1,$path2));
var_dump(get_relative_path($path1,$path3));
var_dump(get_relative_path($path1,$path4));
?>