Anthony Ferrara has shared some thoughts in his latest post about some of the PHP frameworks that have come up lately - ones based in PECL extensions, not in userland code.

In recent months, a number of new frameworks have cropped up for PHP as PECL extensions (Including YAF and PhalconPHP). They promise to provide huge performance gains and lower resource usage for PHP applications. On the surface, they appear to be incredible tools for improving development. But they aren't all they are cracked up to be. In fact, I would argue that they are actually not necessary at all.

He breaks the arguments down into two sections - what you gain by having the framework based in an extension (like performance) and some of the things you give up (like readability, portability and maintainability).

My argument here would be that if you have a site where you can measure meaningful money savings by putting the framework into C (with taking the additional maintenance costs into account), you likely shouldn't be using a framework anyway.