Persistent HTTP GET variables in PHP - php

Question

Let's say I have some code like this
if(isset($_GET['foo']))
//do something
if(isset($_GET['bar']))
//do something else
If a user is at example.com/?foo=abc and clicks on a link to set bar=xyz, I want to easily take them to example.com/?foo=abc&bar=xyz, rather than example.com/?bar=xyz.
I can think of a few very messy ways to do this, but I'm sure there's something cleaner that I don't know about and haven't been able to track down via Google.

Answer 1

Here's one way....
//get passed params
//(you might do some sanitizing at this point)
$params=$_GET;
//morph the params with new values
$params['bar']='xyz';
//build new query string
$query='';
$sep='?';
foreach($params as $name=>$value)
{
$query.=$sep.$name.'='.urlencode($value);
$sep='&';
}

Answer 2

If you are updating the query string you need ot make sure you don't do something like
$qs="a=1&b=2";
$href="$qs&b=4";
$href contains "a=1&b=2&b=4"
What you really want to do is overwrite the current key if you need to .
You can use a function like this. (disclaimer: Off the top of my head, maybe slightly bugged)
function getUpdateQS($key,$value)
{
foreach ($_GET as $k => $v)
{
if ($k != $key)
{
$qs .= "$k=".urlencode($v)."&"
}
else
{
$qs .= "$key=".urlencode($value)."&";
}
}
return $qs
}
View report

Answer 3

Just set the link that changes bar to xyz to also have foo=abc if foo is already set.
$link = ($_GET['foo'] == 'abc') ? 'foo=abc&bar=xyz' : 'bar=xyz';
?>
Click Me

Answer 4

You would have to render out the links with the proper URL querystring to make that happen. This is a design decision that you would need to make on your end depending on how your system is setup.
I have some sites that have this issue, and what I do is setup a querystring global variable that sets the current page data the top of the page request.
Then when I am rendering the page, if I need to make use of the current query string I do something like:
echo '<a href="myurl.php' . querystring . '&bar=foo';
It's not the cleanest, but it all depends on how your system works.

Answer 5

Save some code and use the built-in http_build_query. I use this wrapper in one of my projects:
function to_query_string($array) {
if(is_scalar($array)) $query = trim($array, '? \t\n\r\0\x0B'); // I could split on "&" and "=" do some urlencode-ing here
else $query = http_build_query($array);
return '?'.$query;
}
Also, though it isn't often used, you can have $_GET on the left-hand side of an assignment:
$_GET['overriden_or_new'] = 'new_value';
echo 'Yeah!';
Other than that, just do what Paul Dixon said.