REQUEST-DATA-FORMAT should be NV:
$headerArray = array(
'X-PAYPAL-SECURITY-USERID:'.$payPalUser_Id,
'X-PAYPAL-SECURITY-PASSWORD:'.$payPalPassword,
'X-PAYPAL-SECURITY-SIGNATURE:'.$payPalSig,
'X-PAYPAL-REQUEST-DATA-FORMAT:NV',
'X-PAYPAL-RESPONSE-DATA-FORMAT:JSON',
'X-PAYPAL-APPLICATION-ID:'.$appID
);