To follow PHP Coding standards.
I came across PHP Coding standards through PHP Weblog.
I went through the entire document and it looks pretty solid to me. There are few things which they suggest to do differently then I do, but I think they have good ‘justification’ for the style they advocate. I will now do all my PHP coding following this standard. [It's not like I don't follow any standard, I do follow _some_ kind of standard which comes from my intuition and its not documented! 
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I also encourage you to follow this standard or a similar one so that the person who will read your code later will be happy because it becomes easy for him/her to understand the code! 
JD