The role of the Var manifesto in PHP

What is the function of the Var declaration in PHP?
Recently learned PHP, but even though the PHP Bible involves a wide range of teaching materials also did not mention that PHP can declare variables with VAR, but look at a book on the network, the use of Var to declare the Ming. Such as:

Class test{
var $a;
var $b;
......
}


I tested it as if Var could only be used in the class, and would be an error if used outside of the class (PHP5.3).

What can be summed up at this point, which is the potential system to talk about the other role of this keyword?

------Solution--------------------
Public in PHP5 is equivalent to Var.
Just to be compatible with PHP4, so also keep Var
------Solution--------------------
Forward compatibility

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