PHP variable Constant Definition method

PHP variable Constant Definition method

A description of the definitions for some common static constants under PHP.

1. Defining constants

Define ("CONSTANT", "Hello World");

Constants can only contain scalar data (boolean,integer,float and string).
When calling a constant, simply use the name to get the value of the constant, not the "$" symbol, such as: Echo CONSTANT;
Note: Constants and (global) variables are in different namespaces. This means, for example, that TRUE and $TRUE are different.

2. Common variables

$a = "Hello";

3. Variable variable

(Use two dollar sign ($))

$ $a = "world";

Each of the two variables is defined:
$a content is "hello" and $hello content is "world".
Therefore, it can be expressed as:
echo "$a ${$a}" or echo "$a $hello"; they all output: Hello World
To use mutable variables with arrays, you must address an ambiguous question. This is when you write the $ $a [1], the parser needs to know if you want to $a [1] as a variable, or you want $ $a as a variable and take out the value of the variable indexed to [1]. The syntax for solving this problem is to use ${$a [1] for the first case, and ${$a}[1 for the second case).

4. Static variables

Static $a inside the function = 0;
Note: Assigning a value to a declaration with the result of an expression results in a parse error such as static $a =3+3; (error)
Static variables exist only in the local function domain (inside the function), and after the function is executed, the value of the variable is not lost and can be used for recursive invocation

5. Global Variables

Global variables defined in the function body can be used outside the body, the global variables defined outside the function body cannot be used within the function body, and accessing variables globally can be customized $GLOBALS arrays with special PHP:
such as: $GLOBALS ["b"] = $GLOBALS ["a"] + $GLOBALS ["B"];
A real global variable imported in a function domain with the global statement actually establishes a reference to a global variable.
Global $obj;
Note: The static and global definitions for variables are implemented in an applied manner

6. Assign a value to a variable:

Transfer address assignment (simple reference):

$bar = & $foo; Add & sign to the variable that will be assigned
Changing the new variable will affect the original variable, and this assignment is faster
Note: Only named variables can pass address assignment
Note: if
$bar = & $a;
$bar = & $foo;
Changing the value of $bar can only change the value of the variable foo without changing the value of a (the reference changed)

7.PHP Hyper global Variable $globals

Contains a reference to a globally valid variable that refers to each current script. The array's key is labeled as the name of the global variable. $GLOBALS array exists starting with PHP 3.
$_server: The variable is set by the WEB server or directly associated with the execution environment of the current script. Similar to the old array $HTTP _server_vars array (still valid but opposed to use).
$_get: A variable that is submitted to the script via the HTTP GET method.
$_post: A variable that is submitted to the script via the HTTP POST method.
$_cookie: The variable that is submitted to the script via the HTTP cookie method.
$_files: A variable submitted to the script via an HTTP POST file upload.
File upload form to have enctype= "Multipart/form-data"
$_ENV: Executes the variables that the environment submits to the script.
$_request: The variable that is submitted to the script via the get,post and COOKIE mechanism, so the array is not trustworthy. The presence or absence of all variables contained in the array and the order of the variables are defined by the Variables_order configuration instructions in php.ini. The array does not directly emulate the earlier version of PHP 4.1.0. See Import_request_variables ().
Note: since PHP 4.3.0, the file information in $_files no longer exists in $_request.
$_session: The variable currently registered to the script session.
How to disable Phpinfo ():
In php.ini
Disable_functions = Phpinfo ()
Restart the Web server.
Constants in PHP
Constants can only be used with define (constant name, constant value);
Constants can only contain scalar data (boolean,integer,float and string).
You can simply get the value of the constant by specifying its name, and do not precede the constant with the $ symbol. If the constant name is dynamic, you can also use the function
Constant () to read the value of the constant. A list of all defined constants can be obtained with get_defined_constants ().
Note: Constants and (global) variables are in different namespaces. This means, for example, that TRUE and $TRUE are different.
If an undefined constant is used, PHP assumes that it wants the name of the constant itself, as if it were called with a string (CONSTANT corresponds to "CONSTANT"). An error of E_notice level is issued at this time. See why $w 3sky[bar] is wrong in the manual (unless you define bar as a constant in advance with define ()). If you only want to check if a constant is defined, use the defined () function.
Constants and variables are different:
* Constant preceded by no dollar sign ($);
* Constants can only be defined with the Define () function, not through assignment statements;
* Constants can be defined and accessed anywhere without regard to the rules of variable scope;
* Constants cannot be redefined or undefined once defined;
* The value of a constant can only be scalar.
Defining constants

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