Introduction to PHP constants and system constants _php Tutorial

PHP Changshime is considered case-sensitive. Traditionally, constant identifiers are always capitalized.

The PHP constant name and any other PHP tags follow the same naming conventions. A valid constant name begins with a letter or underscore followed by any letter, number, or underscore. This is expressed in regular expressions: [a-za-z_\x7f-\xff][a-za-z0-9_\x7f-\xff]*.

Copy the Code code as follows:

A valid constant name
Define ("FOO", "something");
Define ("FOO2", "something Else");
Define ("Foo_bar", "something More");

Illegal constant name
Define ("2FOO", "something");

The following definitions are legal, but should be avoided: (Custom constants do not start with __)
Maybe someday php will define a __foo__ magic constant.
This will conflict with your code.
Define ("__foo__", "something");

?>

Summarize:
1. Custom Constants
* Must be defined with function define ()
* The value cannot be changed after the definition is complete
* Use the constant name directly, not the same as the variable in the front plus $s

2 System constants:
File:P hp program filename
Line:P HP program file lines
Php_version: The version number of the current parser
Php_os: Executes the current PHP version of the operating system name
__file__ the file name of the script that is currently being processed.
__LINE__ the current number of rows in the script file currently being processed, as in the previous.

True indicates truth (true).
False indicates a pseudo-value (false).

E_error This constant refers to the most recent error.
E_warning This volume refers to the nearest warning point.
E_parse This constant is a potential problem in parsing syntax.

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