In this new post from Manuel Lemos on the PHPClasses.org blog about some of the performance enhancements that were introduced in the latest PHP releases (the 5.4.x series) including variable access optimization.
PHP 5.4 introduced several performance optimizations. One of them was not discussed much in the PHP community but it may affect the performance of your code depending on the way you write it.
He gets into some of the details surrounding the variable access optimization, pointing out how to get the most out of this improvement. He also does a bit of speculation about future versions of the language, including the possible introduction of "Just In Time" compilers.