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Magic Constants

They're are not really constants at all.

What is a magic constant?

If a constant is an identifier whose value cannot change during the execution of the script, then a magic constant is not a constant at all. Instead think of magic constants as predefined constants to any script which it runs. Some magical constants that change depending on where they are used. A few magical constants include:

Constant Description php.net
__LINE__ The current line number of the file.
__FILE__ The full path and filename of the file with symlinks resolved. If used inside an include, the name of the included file is returned.
__DIR__ The directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(__FILE__). This directory name does not have a trailing slash unless it is the root directory.
__FUNCTION__ The function name.
__CLASS__ The class name. The class name includes the namespace it was declared in (e.g. Foo\Bar). Note that as of PHP 5.4 __CLASS__ works also in traits. When used in a trait method, __CLASS__ is the name of the class the trait is used in.
__TRAIT__ The trait name. The trait name includes the namespace it was declared in (e.g. Foo\Bar).
__METHOD__ The class method name.
__NAMESPACE__ The name of the current namespace.

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Operators in PHP