Detailed explanation of PHP variable principle

This article mainly shares with you a detailed explanation of the principles of PHP variables, hoping to help you better understand and master PHP variables.

1. Variable concept

The so-called variable refers to the amount whose value can change in the program.

The program manages and processes data. During the running of the program, we need to store this data. Variables and constants are used to save the data when the program is running.

Variables usually consist of two parts, variable name and variable value

1.1 In php, to define a variable, use the $ symbol, in the definition Don't worry about what data type this variable is.

1.2 Variable assignment, modification, destruction

Increase means assignment, such as $a = “PHP”

Change, reassign That’s it, check $a = “Mysql”

, delete it by quoting it like $a

, use unset like unset($a)

1.3 What exactly does Unset do? What?

Remove the variable reference and then destroy the variable.

2. Standard definition of variable names

1. Variable names usually consist of letters, numbers and underscores, and do not start with numbers.

2. See the name and know the meaning

3. For a variable name consisting of multiple words, how to separate the words, firstName, first_name. (Do as the Romans do)

☞ Pay attention to the details

$ is not part of the variable name. This is php variable syntax, which means that the following identifier is a variable (special this variable)

When using an undefined variable, a notice error will be reported. You can use isset to check

Variable names are case-sensitive. It is recommended to use underscores

3. Assignment between variables

In PHP, variable assignment defaults to value passing, which is also PHP The most basic way of assignment.

There is another way to pass by value, which is to pass by reference.

☞Attention to details

When unset a variable, delete the variable and the reference between the identifier and the variable

If there is a variable name, only the variable that already exists in the memory satisfies the conditions for reference assignment. ($bar = &(24 * 7); // Illegal;)

4. Variable variable

Variable name (variable identifier), it can also is a variable, this is a mutable variable.

Simple example

5. Predefined variables

There are many variables in PHP that can be used directly without being defined by user scripts, which are called predefined variables.

　$_POST

　$_GET

　$_REQUEST

　$_SERVER

　$_FIELS

　$_SESSION

　$_COOKIE      

  $_ENV

  $GLOBALS

Regarding get and post, if a variable exists in both get and post, which value should be taken. (As agreed in the php.ini configuration)

works after POST.

$_REQUST = $_POST + $_GET

If it is clear whether it is a post or a get submission, just use post or get directly. When it is not clear, you can use $_REQUEST.

$_SERVER, some information related to HTTP protocol and server.

☞ Pay attention to details

When using request, if post and get have the same variable name, only post will be retained, depending on the php configuration request_order = 'GP';

　$GLOBALS refers to all variables available in the global scope

6. Scope of variables

What is scope?

Refers to is the effective scope of the variable.

Global variables (that is, the effective scope, in the current script, will be invalid when the script ends.)

Local variables

Super global

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Detailed explanation of PHP variables and dynamic string insertion into variables

In-depth understanding of PHP variable structure

About PHP variables introduce

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