1. No commented out code.

Commenting out code makes sense if you do not use a revision control system (like CVS/SVN/Git/etc). Then again... why not use a revision control system?
When you do make use of an RCS leaving commented code behind only tends to clutter up files and reduces code readability, especially in those hard times when you're on a console and don't have an editor with syntax highlighting and a big resolution.

2. Write control structures like you would say them

It is very easy to make control structures (if, switch, while, etc...) unreadable.
"Hmm... let's see, this should be... readable... and that should be accessible and this..."
And soon you end up with a random collection of expressions that, to the uninitiated, reads like alphabet soup.
An easy trick to improve readability is to read it to yourself, from start to finish; this forces you to read it anew and clears up strange comparisons like these: '!$apples===false'.

3. Use descriptive variable names

I think this is the hallmark of the inexperienced or just bad programmer. Using variables like $a or $b is making a major sacrifice in readability for a negligible improvement in performance.
Variable names are cheap, programmer time is not.

4. No documentation in functions, only above them.

Documentation in functions is a big warning flag that the code probably needs to be refactored. Code should be readable.
Nearly every time I come across a snippet of code that I want to place a comment above, I stop to think about it and end up moving the code to a separate method with its own phpDoc docblock.

5. Use the identity qualifier (===) and type casting instead of the == qualifier for simple type checking

PHPs type casting is very useful when you are assigning variables, but when you are checking conditions you don't necessarily want your string to suddenly be converted to an integer.
Use ((int)$id===1) instead of ($id==1) so you're not surprised when id contains an object and suddenly your check always returns true.

6. Use single quotes (') by default and double quotes only when you want to put variables in your string

A fun bit of nitpicking: at 2 million string operations, using the single quotes for simple strings instead of the double quote will save you a whopping 0,6 seconds!
It's the little things...

7. No rule is sacred

In the end, I've broken each and every one of my best practices when it suited the project.
The end result (a quality product) is all that matters.