Looking for suggestions on "tightening up" code and what other framework / methodology to adopt

Looking for suggestions on "tightening up" code and what other framework /
methodology to adopt

I have been a PHP / Javascript / mySQL programmer for over 10 years. I
feel like my expertise in PHP has plateaued. For the past four years, I
have been developing web-based apps for the company I work for and have
somewhat mastered jQuery. My deployed apps are PHP OOP and jQuery/UI
dependent. I'm still a bit antiquated as I use mySQLI rather than PDO, but
the apps I have written are very database-heavy and rewriting all that
code would be a brutal task. I recently purchased "Secrets of the
Javascript Ninja" by Resig & Bibeault and am very intrigued by most of the
concepts in the book but, again, it would take massive rewrites to delve
into all of my Javascript and make the changes that they describe. So,
basically, as my title alludes to, I'm seeking not only something new to
learn but something that I can rapidly and non-destructively apply to my
existing code that would improve performance, increase code re-usability
and perhaps expand troubleshooting capabilities. With my current
collection of tools, can anyone suggest any additional framework or
methodology that I should consider adopting? The one overall limitation is
that a large majority of my company's intranet users are restricted to
using IE as their current browser.