In this new post to the PHPWomen site, Kim Rowan shows one way that you can effectively handle concurrency in your applications (in her case, a Symfony app).

Concurrent user activity on the web can take many forms. For example, two online shoppers may simultaneously try to buy the last pair of 'gotta-have-em' shoes in stock. Presumably one potential outcome in this scenario is to place the shoes on back-order for the slower shopper. The concurrency challenge I faced recently, however, was a bit different...

She uses a "last updated" data field in her form to see when the record in question was last changed. When the form is submitted the script checks against the updated date on the record to see if it's later than the one submitted. If it's more recent, the user's request could cause errors, so it fails.