We are creating the task as a centralized user.
We are using the SOAP api via the PHP library. We are using an enterprise WSDL and client

Here an example of the API call we make to create the task:
<?xml version="1.0" encoding="UTF-8"?> <SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"; xmlns:ns1="urn:enterprise.soap.sforce.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><SOAP-ENV:Header><ns1:SessionHeader><ns1:sessionId>00D8E0000008eZd!AQcAQBNHquRZygS9Mie4p2dHvRIWArlv2.AG8nHufSxtHRYsMRlsQl3iq5NZJz6yeJtkR6qPRs7rH_72NKGjhgzJWjOQoA_s</ns1:sessionId></ns1:SessionHeader><ns1:EmailHeader><ns1:triggerAutoResponseEmail>true</ns1:triggerAutoResponseEmail><ns1:triggerOtherEmail>true</ns1:triggerOtherEmail><ns1:triggerUserEmail>true</ns1:triggerUserEmail></ns1:EmailHeader></SOAP-ENV:Header><SOAP-ENV:Body><ns1:create><ns1:sObjects xsi:type="ns1:Task"><Id/><OwnerId>005D00000069MdXIAU</OwnerId><Priority>Normal (default)</Priority><Subject>test task assignment</Subject><Type>Other</Type><WhoId>00Q8E000001Gu2XUAS</WhoId></ns1:sObjects></ns1:create></SOAP-ENV:Body></SOAP-ENV:Envelope>
POST /services/Soap/c/35.0/00D8E0000008eZd/0DF8E000000003j HTTP/1.1 Host: cs87.salesforce.com Connection: Keep-Alive User-Agent: salesforce-toolkit-php/27.0 Accept-Encoding: gzip, deflate Content-Type: text/xml; charset=utf-8 SOAPAction: "" Content-Length: 926