How to use php global variables

How to use php global variables: You can declare the variable through global within the function, the code is [$name = "why";function changeName(){global $name;$name = "what"; }].

How to use php global variables:

1. Definition and use of global variables

Initially, my requirement was as follows:

<?php
$name = "why";
function changeName(){
    $name = "what";
}
changeName();
echo "my name is " . $name . "<br/>";
?>

The result of executing the code is: my name is why, instead of what is displayed after executing changeName(). Analyzing the reason, this is because the $name variable in the function body changeName is set to a local variable by default, and the scope of $name is within changeName. So, modify the code and add global variables as follows:

<?php
global $name;
$name = "why";
function changeName(){
    $name = "what";
}
changeName();
echo "my name is " . $name . "<br/>";
?>

Note: You cannot assign a value to a variable while declaring it global. The usage of global $name = "why" is wrong.

The execution result after defining the global variable is still my name is why. This result surprised me. It turns out that the correct usage of global is: "Introduce an external variable into a function. If the variable is not passed in through a parameter, then it is introduced through global." In other words, when a function refers to an external variable, it can Declare the variable via global within the function so that the variable can be used in the function (equivalent to passing it in as a parameter). Then, modify the code again:

<?php
$name = "why";
function changeName(){
    global $name;
    $name = "what";
}
changeName();
echo "my name is " . $name . "<br/>";
?>

The result of this operation is: my name is what, indicating that global is used to pass parameters, rather than making the scope of the variable global. Look at the following example again:

<?php
$name = "why";
function changeName(){
    global $name;
    $name = "what";
}
function changeName2(){
    $name = "where";
}
changeName();
changeName2();
echo "my name is " . $name . "<br/>";
?>

The execution result is: my name is what. If global $name is added to changeName2(), the execution result is: my name is where.

In summary, the role of global is equivalent to passing parameters. If you want to use a variable declared outside the function, use global to declare the variable, which is equivalent to passing the variable in. , you can reference the variable.

In addition to using the global parameter definition, you can also use the super global variable $GLOBAL:

<?php
$name = "why";
function changeName(){
    $GLOBALS[&#39;name&#39;] = "what";
}
changeName();
echo "my name is " . $name . "<br/>";
?>

2, global and $GLOBAL

An example:

<?php
$var1 = 1;
$var2 = 2;
function test1(){
    $GLOBALS[&#39;var2&#39;] = &$GLOBALS[&#39;var1&#39;];
}
test1();
echo $var2 . "<br />";
$var3 = 1;
$var4 = 2;
function test2(){
    global $var3,$var4;
    $var4 = &$var3;
}
test2();
echo $var4 . "<br />";
?>

The output value of $var2 is 1 and the value of $var4 is 2, because the reference of $var3 points to the reference address of $var4. The actual value of $var4 has not changed. The official explanation is: $GLOBALS['var'] is the external global variable itself, and global $var is the reference or pointer of the same name of the external $var. It means that $GLOBAL is available in all scopes of a script, and you can access them in functions or methods without executing global $variable;. Unlike all other superglobal variables, $GLOBALS is always available in PHP. Another example:

<?php
$var1 = 1;
function test1(){
    unset($GLOBALS[&#39;var1&#39;]);
}
test1();
echo $var1 . "<br />";
$var2 = 1;
function test2(){
    global $var2;
    unset($var2);
}
test2();
echo $var2;
?>

The output result is that $var1 does not exist, and the value of $var2 is 1. This proves that $var2 is just an alias reference, and its value has not been changed in any way. In other words, global $var is actually $var = &$GLOBALS['var'], which is just an alias for calling external variables!

If you want to know more about programming learning, please pay attention to the php training column!

The above is the detailed content of How to use php global variables. For more information, please follow other related articles on the PHP Chinese website!