Note that unlike Java and C++, variables declared inside blocks such as loops or if's, will also be recognized and accessible outside of the block, so:
<?php
for($j=0; $j<3; $j++)
{
if($j == 1)
$a = 4;
}
echo $a;
?>
Would print 4.
Ámbito de las variables
El ámbito de una variable es el contexto dentro del que la variable está definida. La mayor parte de las variables PHP sólo tienen un ámbito simple. Este ámbito simple también abarca los ficheros incluídos y los requeridos. Por ejemplo:
<?php
$a = 1;
include 'b.inc';
?>Aquí, la variable $a estará disponible al interior del script incluido b.inc. Sin embargo, al interior de las funciones definidas por el usuario se introduce un ámbito local a la función. Cualquier variable usada dentro de una función está, por omisión, limitada al ámbito local de la función. Por ejemplo:
<?php
$a = 1; /* ámbito global */
function test()
{
echo $a; /* referencia a una variable del ámbito local */
}
test();
?>Este script no producirá salida, ya que la sentencia echo utiliza una versión local de la variable $a, a la que no se ha asignado ningún valor en su ámbito. Puede que usted note que hay una pequeña diferencia con el lenguaje C, en el que las variables globales están disponibles automáticamente dentro de la función a menos que sean expresamente sobreescritas por una definición local. Esto puede causar algunos problemas, ya que la gente puede cambiar variables globales inadvertidamente. En PHP, las variables globales deben ser declaradas globales dentro de la función si van a ser utilizadas dentro de dicha función.
La palabra clave global
En primer lugar, un ejemplo de uso de global:
Ejemplo #1 Uso de global
<?php
$a = 1;
$b = 2;
function Suma()
{
global $a, $b;
$b = $a + $b;
}
Suma();
echo $b;
?>
El script anterior producirá la salida 3. Al declarar
$a y $b globales dentro de la
función, todas las referencias a tales variables se referirán a la
versión global. No hay límite al número de variables globales que se
pueden manipular dentro de una función.
Un segundo método para acceder a las variables desde un ámbito global es usando el array $GLOBALS. El ejemplo anterior se puede reescribir así:
Ejemplo #2 Uso de $GLOBALS en lugar de global
<?php
$a = 1;
$b = 2;
function Suma()
{
$GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
}
Suma();
echo $b;
?>El array $GLOBALS es un array asociativo con el nombre de la variable global como clave y los contenidos de dicha variable como el valor del elemento del array. $GLOBALS existe en cualquier ámbito, esto ocurre ya que $GLOBALS es una superglobal. Aquí hay un ejemplo que demuestra el poder de las superglobales:
Ejemplo #3 Ejemplo que demuestra las superglobales y el ámbito
<?php
function test_global()
{
// La mayoría de variables predefinidas no son "super" y requieren
// 'global' para estar disponibles al ámbito local de las funciones.
global $HTTP_POST_VARS;
echo $HTTP_POST_VARS['name'];
// Las superglobales están disponibles en cualquier ámbito y no
// requieren 'global'. Las superglobales están disponibles desde
// PHP 4.1.0, y ahora HTTP_POST_VARS se considera obsoleta.
echo $_POST['name'];
}
?>Nota:
Utilizar una clave
globalfuera de una función no es un error. Esta puede ser utilizada aún si el fichero está incluido desde el interior de una función.
Uso de variables static
Otra característica importante del ámbito de las variables es la variable estática. Una variable estática existe sólo en el ámbito local de la función, pero no pierde su valor cuando la ejecución del programa abandona este ámbito. Consideremos el siguiente ejemplo:
Ejemplo #4 Ejemplo que demuestra la necesidad de variables estáticas
<?php
function test()
{
$a = 0;
echo $a;
$a++;
}
?>
Esta función tiene poca utilidad ya que cada vez que es llamada asigna a
$a el valor 0 e imprime un
0. La sentencia $a++, que incrementa la
variable, no sirve para nada, ya que en cuanto la función finaliza, la
variable $a desaparece. Para hacer una función útil
para contar, que no pierda la pista del valor actual del conteo, la
variable $a debe declararse como estática:
Ejemplo #5 Ejemplo del uso de variables estáticas
<?php
function test()
{
static $a = 0;
echo $a;
$a++;
}
?>
Ahora, $a se inicializa únicamente en la primera
llamada a la función, y cada vez que la función test() es llamada,
imprimirá el valor de $a y lo incrementa.
Las variables estáticas también proporcionan una forma de manejar funciones recursivas. Una función recursiva es la que se llama a sí misma. Se debe tener cuidado al escribir una función recursiva, ya que puede ocurrir que se llame a sí misma indefinidamente. Hay que asegurarse de implementar una forma adecuada de terminar la recursión. La siguiente función cuenta recursivamente hasta 10, usando la variable estática $count para saber cuándo parar:
Ejemplo #6 Variables estáticas con funciones recursivas
<?php
function test()
{
static $count = 0;
$count++;
echo $count;
if ($count < 10) {
test();
}
$count--;
}
?>Nota:
Las variables estáticas pueden ser declaradas como se ha visto en los ejemplos anteriores. Desde PHP 5.6 se pueden asignar valores a estas variables que sean el resultado de expresiones, aunque no se pueden usar funciones aquí, lo cual causaría un eror de análisis.
Ejemplo #7 Declaración de variables estáticas
<?php
function foo(){
static $int = 0; // correcto
static $int = 1+2; // correcto (a partir de PHP 5.6)
static $int = sqrt(121); // incorrecto (ya que es una función)
$int++;
echo $int;
}
?>
Nota:
Las declaraciones estáticas son resueltas en tiempo de compilación.
Referencias con variables global y static
El motor Zend 1, utilizado por PHP 4, implementa los modificadores
static y global para variables
en términos de referencias.
Por ejemplo, una variable global verdadera importada dentro del ámbito
de una función con global crea una referencia a la
variable global. Esto puede ser causa de un comportamiento inesperado,
tal y como podemos comprobar en el siguiente ejemplo:
<?php
function prueba_referencia_global() {
global $obj;
$obj = &new stdclass;
}
function prueba_no_referencia_global() {
global $obj;
$obj = new stdclass;
}
prueba_referencia_global();
var_dump($obj);
prueba_no_referencia_global();
var_dump($obj);
?>El resultado del ejemplo sería:
NULL
object(stdClass)(0) {
}
Un comportamiento similar se aplica a static. Las
referencias no son almacenadas estáticamente.
<?php
function &obtener_instancia_ref() {
static $obj;
echo 'Objeto estático: ';
var_dump($obj);
if (!isset($obj)) {
// Asignar una referencia a la variable estática
$obj = &new stdclass;
}
$obj->property++;
return $obj;
}
function &obtener_instancia_no_ref() {
static $obj;
echo 'Objeto estático: ';
var_dump($obj);
if (!isset($obj)) {
// Asignar el objeto a la variable estática
$obj = new stdclass;
}
$obj->property++;
return $obj;
}
$obj1 = obtener_instancia_ref();
$aun_obj1 = obtener_instancia_ref();
echo "\n";
$obj2 = obtener_instancia_no_ref();
$aun_obj2 = obtener_instancia_no_ref();
?>El resultado del ejemplo sería:
Objeto estático: NULL
Objeto estático: NULL
Objeto estático: NULL
Objeto estático: object(stdClass)(1) {
["property"]=>
int(1)
}
Este ejemplo demuestra que al asignar una referencia a una variable
estática, esta no es recordada cuando se invoca la
funcion &obtener_instancia_ref() por segunda vez.
add a note
User Contributed Notes 9 notes
dodothedreamer at gmail dot com ¶
13 years ago
warhog at warhog dot net ¶
18 years ago
Some interesting behavior (tested with PHP5), using the static-scope-keyword inside of class-methods.
<?php
class sample_class
{
public function func_having_static_var($x = NULL)
{
static $var = 0;
if ($x === NULL)
{ return $var; }
$var = $x;
}
}
$a = new sample_class();
$b = new sample_class();
echo $a->func_having_static_var()."\n";
echo $b->func_having_static_var()."\n";
// this will output (as expected):
// 0
// 0
$a->func_having_static_var(3);
echo $a->func_having_static_var()."\n";
echo $b->func_having_static_var()."\n";
// this will output:
// 3
// 3
// maybe you expected:
// 3
// 0
?>
One could expect "3 0" to be outputted, as you might think that $a->func_having_static_var(3); only alters the value of the static $var of the function "in" $a - but as the name says, these are class-methods. Having an object is just a collection of properties, the functions remain at the class. So if you declare a variable as static inside a function, it's static for the whole class and all of its instances, not for each object.
Maybe it's senseless to post that.. cause if you want to have the behaviour that I expected, you can simply use a variable of the object itself:
<?php
class sample_class
{ protected $var = 0;
function func($x = NULL)
{ $this->var = $x; }
} ?>
I believe that all normal-thinking people would never even try to make this work with the static-keyword, for those who try (like me), this note maybe helpfull.
andrew at planetubh dot com ¶
15 years ago
Took me longer than I expected to figure this out, and thought others might find it useful.
I created a function (safeinclude), which I use to include files; it does processing before the file is actually included (determine full path, check it exists, etc).
Problem: Because the include was occurring inside the function, all of the variables inside the included file were inheriting the variable scope of the function; since the included files may or may not require global variables that are declared else where, it creates a problem.
Most places (including here) seem to address this issue by something such as:
<?php
//declare this before include
global $myVar;
//or declare this inside the include file
$nowglobal = $GLOBALS['myVar'];
?>
But, to make this work in this situation (where a standard PHP file is included within a function, being called from another PHP script; where it is important to have access to whatever global variables there may be)... it is not practical to employ the above method for EVERY variable in every PHP file being included by 'safeinclude', nor is it practical to staticly name every possible variable in the "global $this" approach. (namely because the code is modulized, and 'safeinclude' is meant to be generic)
My solution: Thus, to make all my global variables available to the files included with my safeinclude function, I had to add the following code to my safeinclude function (before variables are used or file is included)
<?php
foreach ($GLOBALS as $key => $val) { global $$key; }
?>
Thus, complete code looks something like the following (very basic model):
<?php
function safeinclude($filename)
{
//This line takes all the global variables, and sets their scope within the function:
foreach ($GLOBALS as $key => $val) { global $$key; }
/* Pre-Processing here: validate filename input, determine full path
of file, check that file exists, etc. This is obviously not
necessary, but steps I found useful. */
if ($exists==true) { include("$file"); }
return $exists;
}
?>
In the above, 'exists' & 'file' are determined in the pre-processing. File is the full server path to the file, and exists is set to true if the file exists. This basic model can be expanded of course. In my own, I added additional optional parameters so that I can call safeinclude to see if a file exists without actually including it (to take advantage of my path/etc preprocessing, verses just calling the file exists function).
Pretty simple approach that I could not find anywhere online; only other approach I could find was using PHP's eval().
larax at o2 dot pl ¶
18 years ago
About more complex situation using global variables..
Let's say we have two files:
a.php
<?php
function a() {
include("b.php");
}
a();
?>
b.php
<?php
$b = "something";
function b() {
global $b;
$b = "something new";
}
b();
echo $b;
?>
You could expect that this script will return "something new" but no, it will return "something". To make it working properly, you must add global keyword in $b definition, in above example it will be:
global $b;
$b = "something";
Michael Bailey (jinxidoru at byu dot net) ¶
20 years ago
Static variables do not hold through inheritance. Let class A have a function Z with a static variable. Let class B extend class A in which function Z is not overwritten. Two static variables will be created, one for class A and one for class B.
Look at this example:
<?php
class A {
function Z() {
static $count = 0;
printf("%s: %d\n", get_class($this), ++$count);
}
}
class B extends A {}
$a = new A();
$b = new B();
$a->Z();
$a->Z();
$b->Z();
$a->Z();
?>
This code returns:
A: 1
A: 2
B: 1
A: 3
As you can see, class A and B are using different static variables even though the same function was being used.
gried at NOSPAM dot nsys dot by ¶
8 years ago
In fact all variables represent pointers that hold address of memory area with data that was assigned to this variable. When you assign some variable value by reference you in fact write address of source variable to recepient variable. Same happens when you declare some variable as global in function, it receives same address as global variable outside of function. If you consider forementioned explanation it's obvious that mixing usage of same variable declared with keyword global and via superglobal array at the same time is very bad idea. In some cases they can point to different memory areas, giving you headache. Consider code below:
<?php
error_reporting(E_ALL);
$GLOB = 0;
function test_references() {
global $GLOB; // get reference to global variable using keyword global, at this point local variable $GLOB points to same address as global variable $GLOB
$test = 1; // declare some local var
$GLOBALS['GLOB'] = &$test; // make global variable reference to this local variable using superglobal array, at this point global variable $GLOB points to new memory address, same as local variable $test
$GLOB = 2; // set new value to global variable via earlier set local representation, write to old address
echo "Value of global variable (via local representation set by keyword global): $GLOB <hr>";
// check global variable via local representation => 2 (OK, got value that was just written to it, cause old address was used to get value)
echo "Value of global variable (via superglobal array GLOBALS): $GLOBALS[GLOB] <hr>";
// check global variable using superglobal array => 1 (got value of local variable $test, new address was used)
echo "Value ol local variable \$test: $test <hr>";
// check local variable that was linked with global using superglobal array => 1 (its value was not affected)
global $GLOB; // update reference to global variable using keyword global, at this point we update address that held in local variable $GLOB and it gets same address as local variable $test
echo "Value of global variable (via updated local representation set by keyword global): $GLOB <hr>";
// check global variable via local representation => 1 (also value of local variable $test, new address was used)
}
test_references();
echo "Value of global variable outside of function: $GLOB <hr>";
// check global variable outside function => 1 (equal to value of local variable $test from function, global variable also points to new address)
?>
jameslee at cs dot nmt dot edu ¶
19 years ago
It should be noted that a static variable inside a method is static across all instances of that class, i.e., all objects of that class share the same static variable. For example the code:
<?php
class test {
function z() {
static $n = 0;
$n++;
return $n;
}
}
$a =& new test();
$b =& new test();
print $a->z(); // prints 1, as it should
print $b->z(); // prints 2 because $a and $b have the same $n
?>
somewhat unexpectedly prints:
1
2
dexen dot devries at gmail dot com ¶
7 years ago
If you have a static variable in a method of a class, all DIRECT instances of that class share that one static variable.
However if you create a derived class, all DIRECT instances of that derived class will share one, but DISTINCT, copy of that static variable in method.
To put it the other way around, a static variable in a method is bound to a class (not to instance). Each subclass has own copy of that variable, to be shared among its instances.
To put it yet another way around, when you create a derived class, it 'seems to' create a copy of methods from the base class, and thusly create copy of the static variables in those methods.
Tested with PHP 7.0.16.
<?php
require 'libs.php';
require 'setup.php';
class Base {
function test($delta = 0) {
static $v = 0;
$v += $delta;
return $v;
}
}
class Derived extends Base {}
$base1 = new Base();
$base2 = new Base();
$derived1 = new Derived();
$derived2 = new Derived();
$base1->test(3);
$base2->test(4);
$derived1->test(5);
$derived2->test(6);
var_dump([ $base1->test(), $base2->test(), $derived1->test(), $derived2->test() ]);
# => array(4) { [0]=> int(7) [1]=> int(7) [2]=> int(11) [3]=> int(11) }
# $base1 and $base2 share one copy of static variable $v
# derived1 and $derived2 share another copy of static variable $v
randallfstewart at gmail dot com ¶
9 months ago
Note that the global keyword inside a function does (at least) 2 different things:
1) As stated in the manual, it allows the function to use *the global version* of the variable: "...all references to either variable will refer to *the global version*." [emphasis mine]
2) As not stated in the manual, if the variable does not already exist in the global scope, it is created in the global scope.
For example, in the code below, the variable $A is available in the global scope (after functionA is called), even though it was never declared in the global scope:
<?php
echo "<p>This is A before functionA is called: {$A}.</p>";
functionA();
function functionA(){
global $A;
$A = "Declared as global inside functionA";
} // end fcn callGlobal
echo "<p>This is A after functionA is called: {$A}</p>";
?>
Results:
Notice: Undefined variable: A in /home/essma/public_html/global_test.php on line 3
This is A before functionA is called: .
This is A after functionA is called: Declared as global inside functionA
