Any good resources for properly gauging php developer levels and appropriate income?

[Randy Hicks]

I'm looking for any legitimate resources to help properly gauge the different levels of a php developer and find their appropriate income levels. (income related to the nashville economy) 

I feel like I need to find a way to determine exactly how much I know and don't know concerning my php skills. I feel like I only know what I've had to learn which leads me to believe I'm missing out on a lot!

I know there are various test and certifications, but I've never heard anyone speak about them as if they were worth the time. 

appreciate your time,

randy