How to verify if $_GET exists?

You can use isset function:

if(isset($_GET['id'])) {
    // id index exists
}

You can create a handy function to return default value if index doesn't exist:

function Get($index, $defaultValue) {
    return isset($_GET[$index]) ? $_GET[$index] : $defaultValue;
}

// prints "invalid id" if $_GET['id'] is not set
echo Get('id', 'invalid id');

You can also try to validate it at the same time:

function GetInt($index, $defaultValue) {
    return isset($_GET[$index]) && ctype_digit($_GET[$index])
            ? (int)$_GET[$index] 
            : $defaultValue;
}

// prints 0 if $_GET['id'] is not set or is not numeric
echo GetInt('id', 0);

Normally it is quite good to do:

echo isset($_GET['id']) ? $_GET['id'] : 'wtf';

This is so when assigning the var to other variables you can do defaults all in one breath instead of constantly using if statements to just give them a default value if they are not set.

   if (isset($_GET["id"])){
        //do stuff
    }