如何在PHP中加入文件系统路径字符串?

Is there a builtin function in PHP to intelligently join path strings? The function, given "abc/de/" and "/fg/x.php" as arguments, should return "abc/de/fg/x.php"; the same result should be given using "abc/de" and "fg/x.php" as arguments for that function.Is there a builtin function in PHP to intellige