PHP: A way to manually switch languages?

I have a website that has the following language switching algorithm:

• First, it detects the default browser language (I do not know why? but Chrome always gives something like en-EN,ru,lv, so Chrome's default language always is English, it seems).
• Then it writes the language value into a session variable lang and requests the desired string file (i.e. /assets/includes/en-US/strings.php);
• And every string from this file is being included in the HTML code, so the pure HTML has not any plain text in.

Of course, a default language detection is not the reason to stop - I need a manual language switcher like links (LV | EN | RU). So, what id the possible (and maybe the best) way to switch the language and to overwrite the session variable after user clicks to the desired language?

The best way is the simpliest way :)

$langs = array('LV', 'EN', 'RU');

<?php foreach ($langs as $lang): ?>
    <a href="index.php?lang=<?=$lang;?>"> <?=$lang;?> </a>
<?php endforeach; ?>

so you give the user opportunity to change lang via GET in this example.

Overwrite the session to the sent request:

<?php
if(in_array($_GET['lang'], $langs) {
    $_SESSION['lang'] = $_GET['lang']; // to prevent user to change its session to something you don't want to
}
?>

Afterwards you just interact with this session to display content.

You can use redirection, if you have each page written in different language: (but I guess the logic how to interact with the language you have already implemented from the automatic language detection, but still... let me suggest some ways at fast?)

<?php
if (isset($_SESSION['lang']) && $_SESSION['lang'] !== 'EN') {
    header("Location: mysite.com/".$_SESSION['lang']."/index.php");
    exit;
}
?>

Or, you can use translation method.

All of your translations are in a database under columns with the same names as your $langs array.

So you output the content from this particular column:

SELECT lang_{$_SESSION['lang']} FROM translations WHERE string = '$string';