Properties in Class

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Properties
Class member variables are called "properties". You may also see them referred to using other terms
such as "attributes" or "fields", but for the purposes of this reference we will use "properties". They are
defined by using one of the keywords public, protected, or private, followed by a normal variable
declaration. This declaration may include an initialization, but this initialization must be a constant
value--that is, it must be able to be evaluated at compile time and must not depend on run-time
information in order to be evaluated.
See Visibility for more information on the meanings of public, protected, and private.
Note: In order to maintain backward compatibility with PHP 4, PHP 5 will still accept the
use of the keyword var in property declarations instead of (or in addition to) public,
protected, or private. However, var is no longer required. In versions of PHP from 5.0 to
5.1.3, the use of var was considered deprecated and would issue an E_STRICT warning,
but since PHP 5.1.3 it is no longer deprecated and does not issue the warning.
If you declare a property using var instead of one of public, protected, or private, then PHP
5 will treat the property as if it had been declared as public.
Within class methods the properties, constants, and methods may be accessed by using the form $this- >property (where property is the name of the property) unless the access is to a static property within the context of a static class method, in which case it is accessed using the form self::$property. See Static Keyword for more information.
The pseudo-variable $this is available inside any class method when that method is called from within
an object context. $this is a reference to the calling object (usually the object to which the method
belongs, but possibly another object, if the method is called statically from the context of a secondary
object).