Why do most fields (class members) in Android tutorial start with `m`?

I know about camel case rules, but I'm confused with this m rule. What does it stand for? I'm a PHP developer. "We" use first letters of variables as indication of type, like 'b' for boolean, 'i' for integer and so on.

Is 'm' a Java thing? Does it stand for mobile? mixed?

This notation comes from AOSP (Android Open Source Project) Code Style Guidelines for Contributors:

Follow Field Naming Conventions

• Non-public, non-static field names start with m.
• Static field names start with s.
• Other fields start with a lower case letter.
• Public static final fields (constants) are ALL_CAPS_WITH_UNDERSCORES.

Note that the linked style guide is for code to be contributed to the Android Open Source Project.

It is not a style guide for the code of individual Android apps.

A lot of coding guide lines use m for 'members' of a class. So when you're programming you can see the difference between local and member variables.

What is m prefix?

m stands for member variable or data member. Use m prefix for non-public and non-static fields.

When to Use?

private String mCityName;
private float mTemperature;

When not to Use?

public static int mFirstNumber;
public static final String mDATABASE_NAME;

What I do?

Personally, I don't use it. It makes the code more complicated and chaos the readability. If you are still using Notepad for coding I have no words, but modern IDEs are capable of highlighting and coloring member and local variables or anything else.

Conclusion

Use? "Yes" or "No" is your personal choice.