Send data with cURL by post’s data

Hi can I concatenate my post data?

CURLOPT_POSTFIELDS => "{"contact":{"dataFields":[{"key":"FIRSTNAME","value":"Foo"},{"key":"LASTNAME","value":"Bar"}],"email":"[email protected]","optInType":"Single","emailType":"PlainText"},"consentFields":{"fields":[{"key":"UK_OPT_IN_COUNT","value":"Y"}]}}"

I need to replace Foo with $_POST['firstName'], Bar with $_POST['lastName'], and [email protected] with $_POST['email']