urgente ayuda validar variable NULL

[midespis]

lo he intentado y nada he puesto lo siguiente
$requiredFields = array("userId", "developerId", "developerHash", "timestamp", "layerName", "lat", "alt", "lon", "accuracy", "radius", "SEARCHBOX", "CHECKBOXLIST");

if(is_null($CHECKBOXLIST)){
$CHECKBOXLIST = a
} else {
}

foreach ($requiredFields as $field) {
${$field} = $_GET[$field];
}