I am new to php and learning it from php.net. Can anyone tell in the summary why does that page (http://php.net/manual/en/language.references.arent.php) says What references are not & References are not pointers.
I am beginner so please can anyone explain me in simple and easy words ?
In simple words, references are aliases.
A variable in PHP is stored in two pieces: the name and the value. The name points to the value.
$x = 2;
$y = $x;
$z = &$x;
When $x = 2; is executed, the name x is stored in the symbol table of the current scope and the value 2 is stored in a zval (don't ask, it's the internal name of a data structure that stores a value in PHP).
When $y = $x; is executed, the name y is stored in the symbol table of the current scope and the value of $x (2) is copied into a new zval structure.
When $z = &$x; is executed, the name z is stored in the symbol table of the current scope but a new zval is not created. Instead, z is set to point to the same zval as x.
The memory used by the variables $x, $y and $z looks like this:
+---------+ +---------+
| x | -------------> | 2 |
+---------+ +---------+
^
+---------+ | +---------+
| y | -------------------------> | 2 |
+---------+ | +---------+
|
+---------+ |
| z | --------------------+
+---------+
When a value is passed by reference to a function or a function returns a reference the same things happen, only the names are stored in different symbol tables (remark the "current scope" in the explanation above).
Let' see this code:
function f(& $z) {
$y = $z;
$z = $z + 2;
}
$x = 2;
f($x);
After $x = 2; the memory looks like this:
+---------+ +---------+
| x | -------------> | 2 |
+---------+ +---------+
During the execution of function f(), the memory looks like this:
+===== global ====+
| +---------+ | +---------+
| | x | -------------> | 4 |
| +---------+ | +---------+
+=================+ ^
|
+====== f() ======+ |
| +---------+ | | +---------+
| | y | -------------------------> | 2 |
| +---------+ | | +---------+
| | |
| +---------+ | |
| | z | --------------------+
| +---------+ |
+=================+
y and z are stored in a different symbol table than x and they are removed (together with the entire symbol table that contains them) when the call to f() returns.
When y is removed, its value is also removed because there is no name that points to it any more. But, because the value pointed by z is also pointed by x ($z is an alias), the value is not removed together with z and it survives the function call. f() modifies the value using $z; and this change is visible in the main program through the variable $x.
The things happen in a similar way when a function returns a reference. The function returns a value that is not copied but a new name that points to it is created into the symbols table of the calling code.
I suggest you to study Language C pointers if you have any doubt about what they are.
Anyway, this is a very simplified explanation:
References are not the memory addresses that point variables, but they are only a way to pass a parameter to a function and return that parameter modified if the function alter it.