Principle analysis of php variables

1. The so-called variable refers to the amount whose value can change in the program. Programs manage and process 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. You don’t need to worry about the data type of the variable when defining it.

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. Knowing the meaning after seeing the name

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 uses value transfer by default, which is also the most basic assignment method in PHP.

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

☞ Pay attention to the details

When unset a variable, delete the variable and the space between the identifier and the variable Reference

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

4. Variable variable

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

Simple example

5. There are many unnecessary predefined variables in php Variables that can be used directly when defined by user scripts 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 both post and get have the same variable name, only post will be retained, depending on the configuration of PHP request_order = 'GP';

　$GLOBALS refers to all variables available in the global scope

6. Scope of variables

What is scope?

Refers to 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

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