There still seems to be a lot of confusion over the differences between the two extensions despite the length of time that mysqli has been in the wild. I wanted to briefly review the four major benefits of mysqli.

Here's the list

• Prepared Statements
• Secure MySQL connections
• Multi query
• Object Oriented Interface

He also includes a few more links to further information (besides the explanation for each of the topics listed above) including an article from the Zend Developer Zone and a tutorial covering making the switch to MySQLi.