code: function OpenXML_GetRelativePath($FullPath, $RelativeTo) {
  		
  		$fp = explode('/', $FullPath);
  		$fp_file = array_pop($fp);
  		$fp_max = count($fp)-1;
  		
  		$rt = explode('/', $RelativeTo);
  		$rt_file = array_pop($rt);
  		$rt_max = count($rt)-1;
  		
  		// First different item
  		$min = min($fp_max, $rt_max);
  		while( ($min>=0) && ($fp[0]==$rt[0])  ) {
  			$min--;
  			array_shift($fp);
  			array_shift($rt);
  		}
  
  		$path  = str_repeat('../', count($rt));
  		$path .= implode('/', $fp);
  		if (count($fp)>0) $path .= '/';
  		$path .= $fp_file;
  		
  		return $path;
  		
  	}
docs: Return the path of file $FullPath relatively to the path of file $RelativeTo.
  For example:
  'dir1/dir2/file_a.xml' relatively to 'dir1/dir2/file_b.xml' is 'file_a.xml'
  'dir1/file_a.xml' relatively to 'dir1/dir2/file_b.xml' is '../file_a.xml'