Persistent HTTP GET variables in PHP

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.

From stackoverflow

• 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';
  ?>
  <a href="whatever.php?<?= $link ?>">Click Me</a>
  

• 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.

• 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='&';
  }
  

  Byron Whitlock : Very nice. Although you could have used http_build_query to build the query. bravo.

• 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
   }
  
      <a href="reports.php?<?getupdateqs('name','byron');?">View report</a>
  

• 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:

  $_GET['overriden_or_new'] = 'new_value';
  echo '<a href="'.to_query_string($_GET).'">Yeah!</a>';
  

  Other than that, just do what Paul Dixon said.