That’s a simple array containing another array so you can simply specify multiple indexes for included array:
$mc_name = $_GET['identifiers']['mc']['nick'];
To better understand how it works think of it like assigning each array first to a variable like:
$identifiers = $_GET['identifiers'];
$mc_array = $identifiers['mc'];
$mc_name = $mc_array['nick'];
which will essentially do the same thing at once, without the need to specify multiple variables and arrays.
solved Take value from array which is in array [duplicate]