PHP "magic constant"

PHP provides a large number of predefined constants to any script it runs. However, many constants are defined by different extension libraries. only when these extension libraries are loaded

PHP provides a large number of predefined constants to any script it runs. However, many constants are defined by different extension libraries. they only appear when these extension libraries are loaded, dynamically loaded, or included during compilation. There are seven magic constants whose values change with their positions in the code. For example, the value of _ LINE _ depends on the rows in the script. These special constants are case-insensitive, as follows:

Name	Description
_ LINE __	The current row number in the file. Through editplus, we can intuitively see
_ FILE __	The complete file path and file name. If it is used in a file to be included, the file name to be included is returned. Since PHP 4.0.2,_ FILE __It always contains an absolute path (if it is a symbolic link, it is the absolute path after resolution), and the previous version sometimes contains a relative path.
_ DIR __	The directory where the file is located. If it is used in included files, the Directory of the included files is returned. It is equivalentDirname (_ FILE __). Unless it is the root directory, the name of the directory does not include the slash at the end. (Added in PHP 5.3.0)
_ FUNCTION __	Function name (new PHP 4.3.0 ). Starting from PHP 5, this constant returns the name (case sensitive) when the function is defined ). In PHP 4, the value is always lowercase letters.
_ CLASS __	Class name (new PHP 4.3.0 ). Starting from PHP 5, this constant returns the name (case sensitive) when the class is defined ). In PHP 4, the value is always lowercase letters.
_ METHOD __	Class method name (new PHP 5.0.0 ). Returns the name (case sensitive) when the method is defined ).
_ NAMESPACE __	Name of the current namespace (case sensitive ). This constant is defined during compilation (new in PHP 5.3.0)

Note: PHP magic constants are followed by two underscores.