Constants and variables

--Constants
The definition of a constant uses the Define () function, two ways to get a constant, one is to use the constant name directly, and the second is to use the constant () function. Determines whether a constant has been defined using the defined () function.
Common pre-defined constants:
__FILE__: Master configuration file path
Php_version: Current PHP version
Php_os: Current system type




--Variable
After the PHP4 variable does not need to declare, directly to the variable assignment, the variable name is case-sensitive,
Local variables: Using inside a function
Global variables: Acting on the entire PHP file, using the Global keyword declaration
Static variables: After a function call, any value of its variable can be preserved, and the general variable will be deleted after the call is finished, using the keyword static declaration.
Pre-defined variables: These predefined variables allow you to obtain information about the user's session, the environment of the user's operating system, and so on.
--Variable Management

Isset (): Determines whether the variable is set, returns a Boolean value, sets the variable to NULL, and returns false
Unset (): Canceling a defined variable
Empty (): Check if a variable is not declared or the value is False
--Super Global variables
$_get[]:
$_post[]:
$_cookie[]:
$_env[]: Contains an array of environment variables
$_server[]: Storing an array of Web servers

Constants and variables