Default value of member variables

What is the default value of class member variables in PHP?

Why do I often see:

public static $variable = null;

Wouldn't it be enough:

开发者_C百科

public static $variable;

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It would, but some people either don't know that, or prefer to be explicit.

A common convention is to initialize to null when the programmer relies on that null value and do not initialize to null if that member is written to before being read.

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Setting it to null explicitly makes the initial value clear even to people who don't know every intimate detail of PHP.