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IBillIt for AIM & ARB for Authorize.Net
IBillIt was the first, and is still the best, library certified by Authorize.net for their Automated Recurring Billing (ARB) service.
IBillIt includes complete documentation included, along with sample code, getting started and full reference documentation.
Bonus features: the ability to track line order items; totalling of a variety of amounts such as taxes and shipping; the ability to include custom data; the ability to set default properties to avoid redundant code; duplicate billing information; and MD5 Hash Validation support.