Q:

Use mkdir by going one directory back in PHP

Currently I am creating a dir by using ../ to go one step back 
mkdir("../". "src/new");

this is work fine but is this any other way to give a full path rather then using ../ everywhere

A:

This is work fine but is this any other way to give a full path rather
  then using ../ everywhere. 

Yes.  Just set a PHP configuration variable called something like $BASE_PATH in your PHP and then prepend that to the mkdir like this:
$BASE_PATH = '/full/path/to/your/codebase/here/';

If you want to determine what your site’s base path is, just place this line of code in the main index of your site; such as in index.php:
echo "Your path is: " .  realpath(dirname(__FILE__)) . " <br />";

Then load that page.  Somewhere near the top will be this text:

Your path is: /full/path/to/your/codebase/here/

And that will be the base path to your code on whatever system you are checking this out on. 
Then can take that $BASE_PATH setting and place it in a PHP file named something like local.php.   And then you just make sure all of your PHP files have a line like this near the top of the file like this:
require_once('local.php');

Or this:
require_once('../local.php');

Or ever this:
require_once('../../local.php');

Yes, it seems like you are trading the ../ in your mkdir for this in a config file, but the benefit is you only have to set those require_once calls once in your PHP files.  Once that is done, you can just change $BASE_PATH in one place & not have to worry about it. 
And now when you make calls to the file system in your code, do this:
mkdir($BASE_PATH . ' src/new');

What is nice about a setup like this is that you can make your app portable between environments by just changing $BASE_PATH to match your local environment. 
if ($app_environment == 'local') {
  $BASE_PATH = '/full/path/to/your/codebase/here/';
}
else {
  $BASE_PATH = '/var/www/your/remote/site/codebase/here/';
}

Some people will claim that $_SERVER["DOCUMENT_ROOT"] and such would work, but the reality is this automatted method don’t work well form server to server.  Very inconsistent behavior.  It’s always best to set a $BASE_PATH and then use that rather than fumble around with anything else.