Stay on same page when changing language

When I change the language on the page everything changes and starts from the main page (“index.php?lang=en”) but the question is, when I am in “about.php” I want to stay on the same page “about.php?lang=en”…. who can help me to realize that?

session_start();

if (!isset($_SESSION['lang']))
    $_SESSION['lang'] = 'en';
else if (isset($_GET['lang']) && $_SESSION['lang'] != $_GET['lang'] && !empty($_GET['lang'])) {
    if ($_GET['lang'] == 'en')
        $_SESSION['lang'] = 'en';
    else if ($_GET['lang'] == 'ru')
        $_SESSION['lang'] = 'ru';
}


require_once "languages/" . $_SESSION['lang'] . ".php";

<a href="index.php?lang=en" class="dropdown-item _chng-link">english</a> <a href="index.php?lang=ru" class="dropdown-item _chng-link">russian</a>

Answer

This will give you a good direction and cleaner readable & more secure flow:

<?php
session_start();
$lang = 'en'; // default value
$supportedLanguage = array(
  'en' => 'English',
  'ru' => 'Russian'
);

$lang = (!empty($_SESSION['lang']) && isset($supportedLanguage[$_SESSION['lang']])) ? $_SESSION['lang'] : $lang;
$lang = (!empty($_GET['lang']) && isset($supportedLanguage[$_GET['lang']])) ? $_GET['lang'] : $lang;
$_SESSION['lang'] = $lang;

require_once ("languages/" . $_SESSION['lang'] . ".php");

$languageLinks = '';
foreach ($supportedLanguage as $key => $value) {
  $URL = htmlspecialchars($_SERVER['PHP_SELF'], ENT_QUOTES) . '?lang='.$key;
  $languageLinks .= '<a href="'.$URL.'" class="dropdown-item _chng-link">'.$value.'</a> ';
}