PHP: handle undefined exception -

i have device sends data url, depending on situations 3 4 parameters received, when try data 4th parameter using

$a = $_get["xyz"];

in case when "xyz" not exist function

_get['xyz'];

returns undefined. tried using try catch , initialized $a empty string in catch part did not work either. how can undefined exception handled?

  if(isset($_get["xyz"])) {         $a = $_get["xyz"];   }

Braziel