PHP Notice: Undefined index although using try\catch

use isset function to check if the variable is set or not:

if( isset($_GET['id'])){
    $api = new api($_GET["id"]);
    echo $api -> processRequest();
}

If you want a fast and "dirty" solution, you can use

$api = new api(@$_GET["id"]);

Edit:

Since PHP 7.0 there is a much better and accepted solution: using the null coalescing operator (??). With it you can shorten your code to

$api = new api($_GET["id"] ?? null);

and you don't get a notice because you defined what should happen in the case the variable is not defined.

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