Authorize.net Update recurrent billing using ARB - php

Question

Does Authorize.net ARP API accept XML data if the data field is empty ? Do we need to have the data on all the XML elements ?
I used the following XML for updating the subscription.
<?xml version="1.0" encoding="utf-8"?>
<ARBUpdateSubscriptionRequest xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
<merchantAuthentication>
<name>435345345345</name>
<transactionKey>sdfsdfsdfsdf</transactionKey>
</merchantAuthentication>
<refId>155969e50PGj</refId>
<subscriptionId>234324</subscriptionId>
<subscription>
<amount>75</amount>
<customer>
<id>155969</id>
<email></email>
<phoneNumber></phoneNumber>
</customer>
<billTo>
<firstName></firstName>
<lastName></lastName>
<company></company>
<address></address>
<city></city>
<state></state>
<zip></zip>
<country></country>
</billTo>
<shipTo>
<firstName></firstName>
<lastName></lastName>
<company></company>
<address></address>
<city></city>
<state></state>
<zip></zip>
<country></country>
</shipTo>
</subscription>
</ARBUpdateSubscriptionRequest>
I got the following error :
<?xml version="1.0" encoding="utf-8"?>
<ARBUpdateSubscriptionResponse xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
<refId>155969e50PGj</refId>
<messages>
<resultCode>Error</resultCode>
<message>
<code>E00014</code>
<text>Bill-To First Name is required.</text>
</message>
<message>
<code>E00014</code>
<text>Bill-To Last Name is required.</text>
</message>
</messages>
</ARBUpdateSubscriptionResponse>

Answer 1

ARBUpdateSubscriptionRequest is required other XML in request
<?xml version="1.0" encoding="utf-8"?>
<ARBUpdateSubscriptionRequest xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
<merchantAuthentication>
<name>mytestacct</name>
<transactionKey>112223344</transactionKey>
</merchantAuthentication>
<refId>Sample</refId>
<subscriptionId>100748</subscriptionId>
<subscription>
<payment>
<creditCard>
<cardNumber>4111111111111111</cardNumber>
<expirationDate>2010-08</expirationDate>
</creditCard>
</payment>
</subscription>
</ARBUpdateSubscriptionRequest>