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It's best to declare variables when you first use them to ensure that they are always initialized to some valid value and that their intended use is always apparent. The alternative is typically to declare all variables in one location, typically at the top of the block or, even worse, at the top of a function.
In PHP, a variable does not need to be declared before adding a value to it. PHP automatically converts the variable to the correct data type, depending on its value. After declaring a variable it can be reused throughout the code. The assignment operator ( = ) used to assign value to a variable.
In JavaScript, one should declare all variables at the beginning of the function to mitigate the risk of mistakes related to the fact that the scope of variables is a function.
Note: Unlike other programming languages, PHP has no command for declaring a variable. It is created the moment you first assign a value to it. Think of variables as containers for storing data.
That variable isn't uninitialized, it's just undeclared.
Declaring variables in a class definition is a point of style for readability. Plus you can set accessibility (private or public).
Anyway, declaring variables explicitly has nothing to do with OOP, it's programming-language-specific. In Java you can't do that because variables must be declared explicitly.
If you declare a member inside the class you can set its accessibility e.g
private $varname;
You should always declare your member variables and specify their accessibility within your classes. I like to put this information at the end of the class after my functions.
You should define them as soon as you have enough information to do so. Possibly in the constructor or via setter functions.
It is important to do this because it makes life much easier for people working with your code. They don't have to guess where different properties are coming from or why they're there. Also, most (if not all) IDEs will not pick up on class variables unless you've declared them somewhere. Code completion/hints are one of the many benefits of IDEs and without declaring your variables, you will render that functionality useless.
General OOP paradigm of encapsulation says you should not expose your inner state variables out side that means they should be private, that allows you to change an implementation of your class without need to change the code where you make use of it. It's better practice to initialize variables via constructors and getters and setters method of the class.
In general variables should be initialized as soon as you have enough info to do it properly.
If a class variable needs certain info to be sensibly initialized then that info should be passed to the constructor.
Using PHP's syntax to implicitly declare variables at the point of definition is, IMHO a surefire way to introduce bugs - if your class needs a variable then declare it, and use all of the information hiding that OOP affords you.
As Federico Culloca said "That variable isn't uninitialized, it's just undeclared". Also you didn't define any access modifiers for them so that they behaving like public modifier applied to them.
You may already have known, PHP is a loosely typed language. But a programmer should always follow the best practices and define access modifiers manually. It increases code readability.
You can use private modifier for class level variables and provide accessor and mutator methods (Getters and Setters) for them when needed.
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