Detect Browser Language in PHP and set $locale accordingly

[admin]

I'm trying to achieve, that when someone visits my wordpress page the .po (language-) packages of his pefered language are loaded.
At the moment it is possible to change the language by adding a ?lang= parameter to the URL. But i want the right language to be selected based on the browser language.

My code:

<?php
// start the session 
session_start();
$browserlang = " ".$_SERVER['HTTP_ACCEPT_LANGUAGE'];

// if there's a "lang" parameter in the URL...  
if( isset( $_GET[ 'lang' ] ) ) { 

    // ...set a session variable named WPLANG based on the URL parameter...     
    $_SESSION[ 'WPLANG' ] = $_GET[ 'lang' ]; 

    // ...and define the WPLANG constant with the WPLANG session variable 
    $locale = $_SESSION[ 'WPLANG' ];
    echo 'Based on URL parameter';

// if there isn't a "lang" parameter in the URL...  
} else {

    // if the WPLANG session variable is already set...
    if( isset( $_SESSION[ 'WPLANG' ] ) ) {

        // ...define the WPLANG constant with the WPLANG session variable 
        $locale = $_SESSION[ 'WPLANG' ];
        echo 'Based on session variable';

   // if the WPLANG session variable isn't set...
   } else { 

        // set the WPLANG constant to your default language code is (or empty, if you don't need it)        
        $locale = $browserlang;
        echo 'Should be based on browser language. It is:' . $browserlang;

    } 
};
?>

$locale is used to set the language and select the right .po files.

Now i want $locale to be

<blockquote>
$locale = 'en_US'
</blockquote>

by default, but when someone enters the page that has default language "de", "de_DE", "de_CH" or "de_AT" it should be.

<blockquote>
$locale = 'de_DE'
</blockquote>

The Code im currently using isnt working.

$browserlang = " ".$_SERVER['HTTP_ACCEPT_LANGUAGE'];
echo $browserlang;

Shows me the right language, which is "de_DE", but

$locale = $browserlang

doesnt do anything. On the other hand when i set

$locale = 'de_DE'

it works...

Thank you guys in advance.

<strong>Edit:</strong>

When i use

echo $locale

is says de-DE. Thats very stange, because it doesnt work...

<strong>Edit 2:</strong>

Thats because it must be de_DE (underline) not de-DE (minus)... how to fix that?

<strong>Edit 3:</strong>

Finally it works.