There is a few rules on naming conventions, but specifically, a number can't be the first character of a variable/function name. That being said. I name all of my variables lowercase letter, and then each word, capitalize it. It's commonly known as camel case (numberOneVariableName). I do the same thing with functions and objects, but capitalize all of the first letters of each word (MyFunctionName). This helps me know what is what. I also use a Hungarian notation, but others have been pushing me away from it.
This is a highly individual choice - as you'll (normally) be the one that's looking at the code the most, it is important that you are comfortable with the style.
It is more important that whatever style you use, you use it consistently so that other programmers (and yourself) can always view your code in the same way.
Here are some wikipedia articles regarding the matter:
@stoffe1100:
You should really pick a rule to follow or even make up your own
It is a great practice to keep yourself and your code consistent.
It brings readability and better maintainability
For my own preferences, I really hate this style :