URL中取消设置变量会产生错误

Question

I've recently been developing a multi-lingual website. I've got a slight problem though. Every time a button is clicked the language variable needs to change. i did this using the anchor tag (i.e <a href="?lang=En">English</a> ).

The problem arises when other variables besides the language are added to the URL. I would like to redirect the page without getting rid of other variables and just changing the lang variable. So if the url contains "var1=value&var2=value&lang=En", I would like to alter the lang variable and keep the rest as they are. The lang variable can have 3 values: En, Az, Ru.

The method I tried so far:

function URI_ADD_AZ(){
    $URI = $_SERVER['REQUEST_URI'];

    if(isset($_GET['lang'])){
        $lang = $_GET['lang'];
        unset($lang);
    }
    $new_URI = $URI . '?lang=Az';
    return $new_URI;
}

<a href="<?php URI_ADD_AZ?>">Azeri</a>

The problem:

Everytime the button is clicked the lang variable just gets added to the url not altered:

/?lang=Az?lang=Az?lang=Az?lang=Az

How can I make sure it does not keep getting repeated and avoid redirect loops?

Answer 1

The URI_ADD_AZ function posted in the question does not overwrite or remove preexisting occurrences of lang=* in the Query String, therefore duplication of "langs" in the URL. Also there is no handling of the requirement for ? or & depending on location of the key=value pair in the query string.

Here, to simplify things and limit the working string and therefore potential for introducing errors, the $_SERVER var QUERY_STRING is pulled, rather than the entire REQUEST_URI, and PHP_SELF is then prepended to the HREF value.

First thing here is to remove the lang=* including the & depending on it's position. A conditional reference is used to remove the trailing & only if the match is found at the beginning of the string.

Next $lang is retrieved from the $_GET var and validated. And if there's a valid $lang it is appended to the query string taking into consideration whether & is needed or not.

Finally, if not empty, the resulting query string is prepended with ? and returned.

function URI_ADD_AZ() {
    $QS = preg_replace('/((^)|&)lang=[^&]*(?(2)&)?/', '', $_SERVER['QUERY_STRING']);
    $lang = ( isset($_GET['lang']) && in_array($_GET['lang'], array('En','Az','Ru')) )? 'lang=' . $_GET['lang']: '';
    if ( '' != $lang ) {
      if ( '' == $QS ) { $QS = $lang; }
      else { $QS .= "&$lang"; }
    }
    if ( '' != $QS ) {
        return '?' . $QS;
    }
}
<a href="<?php echo $_SERVER['PHP_SELF'] . URI_ADD_AZ(); ?>">Azeri</a>