I’m not familiar with Magento, so it may be best to raise that query on a support forum for their software directly.
A simple HTTP POST call to the relevant endpoint however should do the trick (rather than SOAP). The end point depends on whether you’re using XML, JSON and it depends on the API version and function (all outlined in the API docs).
There’s a PHP example here with the XML output, or I recently posted a PHP example with JSON for another user, here