It should be like this :
$credentialsHeader = new SoapHeader("urn:ebay:apis:eBLBaseComponents", "Credentials",
array(
"Username" => "username.user.name",
"Password" => "password",
"Signature" => "signature",
"Subject" => "my_email_id"
));
$requesterCredential = new SoapHeader("urn:ebay:api:PayPalAPI", "RequesterCredentials", $credentialsHeader);
$client->__setSoapHeaders(array($requesterCredential));