In-depth analysis of the specific use of PHP variables_PHP tutorial

We are using PHP variables 1. Define constants define("CONSTANT", "Hello world.");

Constants can only contain Scalar data (boolean, integer, float and string).
When calling a constant, you only need to simply use the name to get the value of the constant, without adding the "$" symbol, such as: echo CONSTANT;
Note: Constants and (global) variables are in different name spaces. This means for example TRUE and $TRUE are different.

PHP variable 2. Normal variable $a = "hello";

PHP variable 3. Variable variable (use two dollar signs ($) )

$$a = "world";

Both variables are defined:

The content of $a is "hello" and the content of $hello The content is "world".

Therefore, it can be expressed as:

echo "$a ${$a}"; or echo "$a $hello"; both of them will output: hello world
To be able to Variables are used for arrays and must solve an ambiguous problem. This is when writing $$a[1], the parser needs to know whether it wants $a[1] as a variable, or whether it wants $$a as a variable and extracts the variable with index [1] value. The syntax to solve this problem is to use ${$a[1]} for the first case and ${$a}[1] for the second case.

PHP variables 4. Static variables

static $a = 0 inside the function;
Note: Assigning it with the result of an expression in the declaration will result in Parsing errors such as static $a =3+3; (error)
Static variables only exist in the local function domain (inside the function). After the function is executed, the variable value will not be lost and can be used for recursive calls

PHP variables 5. Global variables

Global variables defined within the function body can be used outside the function. Global variables defined outside the function cannot be used within the function body and are in the global scope. To access variables, you can use special PHP to customize the $GLOBALS array:
For example: $GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"];
Used in a function domain A real global variable imported by the global statement actually establishes a reference to the global variable

global $obj;

Note: The static and global definitions of variables are in an application manner Implemented

PHP variable 6. Assign value to variable: Pass address assignment (simple reference):

$bar = &$foo; //Add & symbol to the next

Before the assigned variable, changing the new variable will affect the original variable. This assignment operation is faster
Note: Only named variables can be assigned addresses
Note: If

$bar = &$a;
$bar = &$foo;

Changing the value of $bar can only change the value of variable foo, but not the value of a (the reference has changed)

PHP Variables 7.PHP superglobal variables $GLOBALS:

Contains a reference to each variable that is valid in the global scope of the current script. The keys of this array are the names of global variables. The $GLOBALS array exists since PHP 3.

$_SERVER: Variables are set by the web server or directly associated with the execution environment of the current script. Similar to the old $HTTP_SERVER_VARS array (still valid, but deprecated).
$_GET: Variables submitted to the script via the HTTP GET method.
$_POST: Variables submitted to the script via the HTTP POST method.
$_COOKIE: Variable submitted to the script via the HTTP Cookies method.
$_FILES : Variables submitted to the script via HTTP POST file upload.
The file upload form must have enctype="multipart/form-data"
$_ENV: Variables submitted by the execution environment to the script.
$_REQUEST: Variables submitted to the script via GET, POST and COOKIE mechanisms, so this array is not trustworthy. The presence, absence, and order of all variables contained in this array are defined according to the variables_order configuration directive in php.ini. This array does not directly emulate earlier versions of PHP 4.1.0. See import_request_variables(). NOTE: As of PHP 4.3.0, file information in $_FILES is no longer present in $_REQUEST.
$_SESSION: PHP variable currently registered for the script session.

http://www.bkjia.com/PHPjc/446211.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/446211.htmlTechArticleWe are using PHP variables 1. Define constants define(CONSTANT, Hello world.); Constants can only contain scalar data (boolean, integer, float and string). When calling a constant, just simply...