[php] Best way to get hostname with php

I have a php application that is installed on several servers and all of our developers laptops. I need a fast and reliable way to get the server's hostname or some other unique and reliable system identifier. Here's what we have thought of so far:

<? $hostname = (!empty($_ENV["HOSTNAME"])) ? $_ENV["HOSTNAME"] : env('HOSTNAME'); ?>

<? $hostname = gethostbyaddr($_SERVER['SERVER_ADDR']); ?>

<? $hostname = exec('hostname'); ?>

What do you think?

You could also use...

$hostname = getenv('HTTP_HOST');

php_uname but I am not sure what hostname you want the hostname of the client or server.

plus you should use cookie based approach

The accepted answer gethostname() may infact give you inaccurate value as in my case

gethostname()         = my-macbook-pro     (incorrect)
$_SERVER['host_name'] = mysite.git         (correct)

The value from gethostname() is obvsiously wrong. Be careful with it.

Update as corrected by the comment

Host name gives you computer name, not website name, my bad. My result on local machine is

gethostname()         = my-macbook-pro     (which is my machine name)
$_SERVER['host_name'] = mysite.git         (which is my website name)

I am running PHP version 5.4 on shared hosting and both of these both successfully return the same results:

php_uname('n');

gethostname();

For PHP >= 5.3.0 use this:

$hostname = gethostname();

For PHP < 5.3.0 but >= 4.2.0 use this:

$hostname = php_uname('n');

For PHP < 4.2.0 use this:

$hostname = getenv('HOSTNAME'); 
if(!$hostname) $hostname = trim(`hostname`); 
if(!$hostname) $hostname = exec('echo $HOSTNAME');
if(!$hostname) $hostname = preg_replace('#^\w+\s+(\w+).*$#', '$1', exec('uname -a'));