SPIP add GET variable to URL

I'm using SPIP with files of my own that are included according to keywords. For example, adding the keyword inc-bla to an article makes that the file .../squellettes/.../inc-bla.html is included. In one of those files, I want to make a link to the current page with a GET variable added to the URL:

[(#ENV{date}|diffdays|<{40}|?{<a href="#URL_ARTICLE?date=[(#ENV{date}|next_sunday)]">Some text...</a>,''})]

Now the problem is that the question mark ? is hard-coded, and sometimes the URL already has a question mark. So this code creates links like ...spip.php?article123?date=2014-03-30, which should be ...spip.php?article123&date=2014-03-30. I cannot hard-code an ampersand & because not every link has a question mark in it already.

Is there a way to add a GET variable to a URL in SPIP?

Solution

The right way is using filters. We need to create our own filter, a PHP function like this:

function url_add_get_var($url,$name,$value) {
    $name = urlencode($name);
    $value = urlencode($value);
    // If there is no question mark in the URL ...
    if (strpos($url,'?') === false) {
        // Add the variable with a question mark
        return "$url?$name=$value";
    } else {
        // If there is a question mark, add the variable with an ampersand
        return "$url&$name=$value";
    }
}

Now, we can use it in SPIP like this:

[(text|filter{var1,var2, ...})]

So, in this case:

[(#URL_ARTICLE|url_add_get_var{date,[(#ENV{date}|next_sunday)]})]

And the full code:

[(#ENV{date}|diffdays|<{40}|?{<a href="[(#URL_ARTICLE|url_add_get_var{date,[(#ENV{date}|next_sunday)]})]">Some text...</a>,''})]