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Source Code Fundamentals: Constants

A constant is a named value. Once defined, the value of the constant cannot be changed. A constant can be defined in one of two ways: using the const keyword at the top level, or inside a class or interface. For example:

const int MAX_COUNT = 123;
class C {
  const float MAX_HEIGHT = 10.5;
  const float UPPER_LIMIT = C::MAX_HEIGHT;
}

<<__EntryPoint>>
function main(): void {
  echo "MAX_COUNT = ".MAX_COUNT."\n";
  echo "MAX_HEIGHT = ".C::MAX_HEIGHT."\n";
}
Output 
MAX_COUNT = 123
MAX_HEIGHT = 10.5

Context-Dependent Constants

The following constants --- sometimes referred to as magic constants --- are automatically available to all scripts; their values are not fixed:

Constant Name Description
__CLASS__ string; The name of the current class. From within a trait method, the name of the class in which that trait is used. If the current namespace is a defined, named namespace (not in root), the namespace name and "\" are prepended, in that order. If used outside all classes, the value is the empty string.
__DIR__ string; The directory name of the script. A directory separator is only appended for the root directory.
__FILE__ string; The full name of the script.
__FUNCTION__ string; Inside a function, the name of the current function exactly as it was declared, with the following prepended: If a named namespace exists, that namespace name followed by "". If used outside all functions, the result is the empty string. For a method, no parent-class prefix is present. (See __METHOD__ and anonymous functions.)
__LINE__ int; the number of the current source line
__METHOD__ string; Inside a method, the name of the current method exactly as it was declared, with the following prepended, in order: If a named namespace exists, that namespace name followed by ""; the parent class name or trait name followed by ::. If used outside all methods, the result is the same as for __FUNCTION__.
__NAMESPACE__ string; The name of the current namespace exactly as it was declared. For the root namespace, the result is the empty string.
__TRAIT__ string; The name of the current trait. From within a trait method, the name of the current trait. If used outside all traits, the result is the empty string.

Core Predefined Constants

Namespace HH\Lib\Math contains a number of integer-related constants (such as INT64_MAX and INT64_MIN).

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