This

A class is a mold for an object: it specifies how the object looks like (variables) and what it can do (functions).

If you instanciate a class: you create an object. If you create the class, you can use "this" to refer to the object itsself. This is why you can't set the "this", because it's related to the object. It's a special, read-only variable.

No. this keyword in C++ is a reference to the object at hand, and is actually explicitly passed to the member functions at the ABI level. Explicitly passing a pointer to the object (as the first parameter) in functions is the best equivalent in C. Note that this means

struct foo {
    int   value;
    int (*func)(struct foo *, int);
};

void foo_bar(struct foo *f, int value)
{
    f->value = value;
}

i.e. the pointer to the object is passed as the first parameter, rather than the structure itself. This makes it explicit that the object is passed by reference, and makes understanding and maintaining such code easier.

It is not sane to expect features seen in one programming language to be valid in some other programming language, even if the two are related somehow.

You see, each programming language has their own approach to problem solving, their own paradigm. Because there is no universal best paradigm possible, problems are best solved using a programming language that has the most applicable/efficient/useful paradigm. For example, you don't write a C++ program to expedite common command-line tasks; you use a shell script or other simple scripting language instead.

As a programmer, having the ability to switch from one programming language paradigm to another means you have the ability to look at a problem from different viewpoints. Looking at current software projects, the most robust, vital, and efficient ones are written by programmers who have that ability.

This is why I stated, above, that it is not sane to expect the features or paradigm of one programming language to be portable to others. You should not do that, because it is equivalent to having a single tool, and looking at all problems as if your tool at hand is the only possible tool in solving them. (If all you have is a hammer, all problems start looking like nails.) Learning, and especially learning to accept, the different paradigms in different programming languages, makes for a better programmer, better problem-solver.

There shouldn't be any difference in performance. Its purely a style decision.

  //      name= name; this is also correct

This is not correct. It'll assign your parameter to itself. By using the this keyword, you're declaring which name you're using (i.e. the field on your Employee object).

You may wish to name the field differently from the parameter. However this means that all works well until someone automatically refactors your code to (perhaps inadvertently) declare the field and parameter as the same name!

For this reason you'll often see method signatures defined thus:

public Employee(final String name)

The final keyword prevents reassignment, and stops you from mistakenly reassigning the input parameter, and consequently not assigning to your field. Note also that if you declare the field as final, then compilation will fail if you don't make an assignment to that field. Using final is a good way to trap such errors and also enforce the immutability of an object (often a good thing - it contributes to a more reliable solution, especially in a threaded environment).

$this refers to the current object

Manual says:

The pseudo-variable $this is available when a 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).

Little example:

class Test
{
    private $var;

    public function func()
    {
        $this->var = 1;
        return $this->var;
    }
}

$obj = new Test();

$obj->func();

The this keyword is primarily used in three situations. The first and most common is in setter methods to disambiguate variable references. The second is when there is a need to pass the current class instance as an argument to a method of another object. The third is as a way to call alternate constructors from within a constructor.

Case 1: Using this to disambiguate variable references. In Java setter methods, we commonly pass in an argument with the same name as the private member variable we are attempting to set. We then assign the argument x to this.x. This makes it clear that you are assigning the value of the parameter "name" to the instance variable "name".

public class Foo
{
    private String name;

    public void setName(String name) {
        this.name = name;
    }
}

Case 2: Using this as an argument passed to another object.

public class Foo
{
    public String useBarMethod() {
        Bar theBar = new Bar();
        return theBar.barMethod(this);
    }

    public String getName() {
        return "Foo";
    }
}

public class Bar
{
    public void barMethod(Foo obj) {
        obj.getName();
    }
}

Case 3: Using this to call alternate constructors. In the comments, trinithis correctly pointed out another common use of this. When you have multiple constructors for a single class, you can use this(arg0, arg1, ...) to call another constructor of your choosing, provided you do so in the first line of your constructor.

class Foo
{
    public Foo() {
        this("Some default value for bar");

        //optional other lines
    }

    public Foo(String bar) {
        // Do something with bar
    }
}

I have also seen this used to emphasize the fact that an instance variable is being referenced (sans the need for disambiguation), but that is a rare case in my opinion.