PHP: checking for $_GET variables with if statements

Array Merge

If you're just setting defaults, you could use array_merge():

$defaults = array(
    'getvar1' => 'default setting', //a default value
    'getvar2' => false //now you can just test
);

$params = array_merge($defaults, $_GET); //any missing elements get the default value

echo $params['getvar1']; //will output 'default setting' if $_GET['getvar1'] was not set

if($params['getvar2']){
     //do something if $_GET['getvar2'] was set to a non-false value
}

It should be noted that this only works with things like $_GET - that are already arrays. The ternary method, a getOrDefault() function, or some enigmatic logic operator code is better suited when setting defaults not already in an array.

Logic Operators

Here's an example of using logic operators (similar to using ternary operator) from the default Zend MVC index.php:

defined('APPLICATION_PATH')
    || define('APPLICATION_PATH', realpath(dirname(__FILE__) . '/../application'));

Using the question's example, it would look something like this:

isset($_GET['getvar1']) || $_GET['getvar1'] = 'default value';

How about writing a function like this:

function getOrDefault($index, $default)
{
    return (!isset($_GET[$index]) || empty($_GET[$index]) ? $default : $_GET[$index]);
}

$getVar1 = getOrDefault('getvar1', '');
$getVar2 = getOrDefault('getvar2', '');

This function should be as general as possible and be located somewhere in a function-library. I don't know about the rest of your project. But for example if you do have some kind of request object (containing all information about the current (http)request), you could write specialized versions:

class Request()
{
    public function getPost($param, $default = '')
    {
        if(!isset($_POST[$param]) || empty($_POST[$param])
        {
            return $default;
        }
        return $_POST[$param];
    }

    // If param is set both in $_GET and $_POST, $_GET is return.
    public function getParam($param, $default = '')
    {
        if(!isset($_GET[$param]) || empty($_GET[$param))
        {
            return $this->getPost($param, $default);
        }
        return $_GET[$param];
    }
}

$getVar1 = $request->getParam('getVar1', '');
$getVar2 = $request->getParam('getVar2', '');