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PHP4 does not support the CURLOPT_HTTPAUTH option of curl_setopt function. While most hosting companies provide support for PHP 5, some hosting plans such as Yahoo Small Business Hosting are still using PHP 4.
In order to implement authentication with your XML request in PHP 4 you must set the header information manually. We do not provide any support for implementing this in PHP 4 however I will provide a link to a resource which may start you in the right direction. We have not tested this code and do not guarantee its functionality in any way.
PHP 4 Authentication Example
http://us3.php.net/manual/en/features.http-auth.php
Information about Basic Authentication
http://developer.constantcontact.com/doc/authenticationBasic
 
**NOTE the code provided in the resource linked above utilizes 'SUPER GLOBAL' variables that were not implemented until PHP v4.1.0.