Vale ya encontre que pasa. Mirate esta pagina te ayudara con los arrays:
http://www.ignside.net/man/php/arrays.php 
Código PHP:
$requiredFields = array("userId"=>"", "developerId"=>"", "developerHash"=>"", "timestamp"=>"", "layerName"=>"", "lat"=>"", "alt"=>"", "lon"=>"", "accuracy"=>"", "radius"=>"", "SEARCHBOX"=>"", "CHECKBOXLIST"=>"");
if(empty($requiredFields['CHECKBOXLIST'])){
$requiredFields['CHECKBOXLIST'] = "a";
echo $requiredFields['CHECKBOXLIST'];
} else {
echo "requiredFields['CHECKBOXLIST'] contiene: ".$requiredFields['CHECKBOXLIST'];
}