Magic constants are the predefined constants in PHP which get changed on the basis of their use. They start with double underscore (__) and ends with double underscore.
They are similar to other predefined constants but as they change their values with the context, they are called magic constants.
There are nine magic constants in PHP. In which eight magic constants start and end with double underscores (__).
All of the constants are resolved at compile-time instead of run time, unlike the regular constant. Magic constants are case-insensitive.
All the constants are defined below with the example code:
It returns the current line number of the file, where this constant is used.
This magic constant returns the full path of the executed file, where the file is stored. If it is used inside the include, the name of the included file is returned.
It returns the full directory path of the executed file. The path returned by this magic constant is equivalent to dirname(__FILE__). This magic constant does not have a trailing slash unless it is a root directory.
This magic constant returns the function name, where this constant is used. It will return blank if it is used outside of any function.
It returns the class name, where this magic constant is used. __CLASS__ constant also works in traits.
This magic constant returns the trait name, where it is used.
It returns the name of the class method where this magic constant is included. The method name is returned the same as it was declared.
It returns the current namespace where it is used.
This magic constant does not start and end with the double underscore (__). It returns the fully qualified name of the ClassName. ClassName::class is added in PHP 5.5.0. It is useful with namespaced classes.
Note: Remember namespace must be the very first statement or after any declare call in the script, otherwise it will generate Fatal error.