PHP在chdir（）中使用变量？

Using PHP,

chdir('../) works completely fine. However, when going the same path just having a declared variable instead, it prompts an error saying:

Warning: chdir(): No such file or directory (errno 2)

$var = "../";
chdir($var)

I have tried declaring $var with 's instead, and also tried to use " inside chdir but both cases gives same error.

why is this?

Try:

    $var = __DIR__ . DIRECTORY_SEPARATOR . "..";
    chdir($var);