#Constants

##General

A *constant* is a [named](09-lexical-structure.md#names) value. Once defined, the value
of the constant can not be changed.

A constant can be defined in one of two ways: as a *c-constant* using a
[*const-declaration*](14-classes.md#constants), or as a *d-constant* by calling the library
function [`define`](http://www.php.net/define). However, the two approaches differ slightly.
Specifically:

-   The name of a c-constant must comply with the lexical grammar for a
    name while that for a d-constant can contain any source character.
-   The name of a c-constant is case-sensitive while that for a
    d-constant can be case-sensitive or case-insensitive based on the
    value of the third argument passed to `define`.
-   If `define` is able to define the given name, it returns `TRUE`;
    otherwise, it returns `FALSE`.

The constants can only hold a value of a [scalar type](05-types.md#scalar-types), an array or a [resource](05-types.md#resource-types).

The library function [`defined`](http://www.php.net/defined) reports if a given name (specified as
a string) is defined as a constant. The library function [`constant`](http://www.php.net/constant)
returns the value of a given constant whose name is specified as a
string.

**Examples**

```PHP
const MAX_HEIGHT = 10.5;              // define two (case-sensitive) c-constants
const UPPER_LIMIT = MAX_HEIGHT;
define('COEFFICIENT_1', 2.345, TRUE); // define a case-insensitive d-constant
define('FAILURE', FALSE, FALSE);      // define a case-sensitive d-constant
```

##Context-Dependent Constants

The following constants—sometimes referred to as *magic constants*—are
automatically available to all scripts; their values are not fixed and they are case-insensitive:

 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 other than the default, the namespace name and `\` are prepended, in that order. If used outside all classes, the value is the empty string.
`__COMPILER_HALT_OFFSET__` |  `int`; When the [`__halt_compiler();`](04-basic-concepts.md#__halt_compiler) construct is used, this constant contains the byte offset in the source file immediately following the `__halt_compiler();` token in this file.
`__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](13-functions.md#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 default 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.

Constant names beginning with __ are reserved for future use by the Engine.

##Core Predefined Constants

The following constants are automatically available to all scripts; they are case-sensitive with the exception of `NULL`, `TRUE` and `FALSE`:

Constant Name | Description
-------------   | -----------
`DEFAULT_INCLUDE_PATH` |  `string`; the [`fopen`](http://www.php.net/fopen) library function include path is used if it is not overridden by the `php.ini` setting `include_path`.
`E_ALL` | `int`; All errors and warnings, as supported.
`E_COMPILE_ERROR` | `int`; Fatal compile-time errors. This is like an `E_ERROR`, except that `E_COMPILE_ERROR` is generated by the scripting engine.
`E_COMPILE_WARNING` | `int`; Compile-time warnings (non-fatal errors). This is like an `E_WARNING`, except that `E_COMPILE_WARNING` is generated by the scripting engine.
`E_CORE_ERROR` |  `int`; Fatal errors that occur during PHP's initial start-up. This is like an `E_ERROR`, except that `E_CORE_ERROR` is generated by the core of PHP.
`E_CORE_WARNING` |  `int`; Warnings (non-fatal errors) that occur during PHP's initial start-up. This is like an `E_WARNING`, except that `E_CORE_WARNING` is generated by the core of PHP.
`E_DEPRECATED` |  `int`; Deprecation notices. Enable this to receive warnings about code that will not work in future versions.
`E_ERROR` | `int`; Fatal run-time errors. These indicate errors that cannot be recovered from, such as a memory allocation problem. Execution of the script is halted.
`E_NOTICE` | `int`; Run-time notices. Indicate that the script encountered something that could indicate an error, but could also happen in the normal course of running a script.
`E_PARSE` | `int`; Compile-time parse errors.
`E_RECOVERABLE_ERROR` | `int`; Catchable fatal error. It indicates that a probably dangerous error occurred, but did not leave the Engine in an unstable state. If the error is not caught by a user defined handler (see the library function [`set_error_handler`](http://www.php.net/set_error_handler)), the application aborts as it was an `E_ERROR`.
`E_STRICT` |  `int`; Have PHP suggest changes to the source code to ensure the best interoperability.
`E_USER_DEPRECATED` | `int`; User-generated error message. This is like an `E_DEPRECATED`, except that `E_USER_DEPRECATED` is generated in PHP code by using the library function [`trigger_error`](http://www.php.net/trigger_error).
`E_USER_ERROR` |  `int`; User-generated error message. This is like an `E_ERROR`, except that `E_USER_ERROR` is generated in PHP code by using the library function [`trigger_error`](http://www.php.net/trigger_error).
`E_USER_NOTICE` | `int`; User-generated warning message. This is like an `E_NOTICE`, except that `E_USER_NOTICE` is generated in PHP code by using the library function [`trigger_error`](http://www.php.net/trigger_error).
`E_USER_WARNING` |  `int`; User-generated warning message. This is like an `E_WARNING`, except that  `E_USER_WARNING` is generated in PHP code by using the library function [`trigger_error`](http://www.php.net/trigger_error).
`E_WARNING` | `int`; Run-time warnings (non-fatal errors). Execution of the script is not halted.
`E_USER_DEPRECATED` | `int`; User-generated warning message. This is like an `E_DEPRECATED`, except that `E_USER_DEPRECATED` is generated in PHP code by using the library function [`trigger_error`](http://www.php.net/trigger_error).
`FALSE` |   `bool`; the case-insensitive Boolean value `FALSE`.
`INF` | `float`; Infinity
`M_1_PI` |  `float`; 1/pi
`M_2_PI` |  `float`; 2/pi
`M_2_SQRTPI` |  `float`; 2/sqrt(pi)
`M_E` | `float`; e
`M_EULER` | `float`; Euler constant
`M_LN10` |  `float`; log_e 10
`M_LN2` | `float`; log_e 2
`M_LNPI` |  `float`; log_e(pi)
`M_LOG10E` |  `float`; log_10 e
`M_LOG2E` | `float`; log_2 e
`M_PI` |  `float`; Pi
`M_PI_2` |  `floa`t; pi/2
`M_PI_4` |  `float`; pi/4
`M_SQRT1_2` | `float`; 1/sqrt(2)
`M_SQRT2` | `float`; sqrt(2)
`M_SQRT3` | `float`; sqrt(3)
`M_SQRTPI` |  `float`; sqrt(pi)
`NAN` | `float`; Not-a-Number
`NULL` |  `null`; the case-insensitive value `NULL`.
`PHP_BINARY` |  `string`; the PHP binary path during script execution.
`PHP_BINDIR` |  `string`; the installation location of the binaries.
`PHP_CONFIG_FILE_PATH` |  `string`; location from which php.ini values were parsed
`PHP_CONFIG_FILE_SCAN_DIR` |  `string`; The directory containing multiple INI files, all of which were parsed on start-up.
`PHP_DEBUG` | `int`; Indicates whether the engine was built with debugging enabled.
`PHP_EOL` | `string`; the end-of-line terminator for this platform.
`PHP_EXTENSION_DIR` | `string`; The directory to be searched by the library function [`dl`](http://www.pph.net/dl) when looking for runtime extensions.
`PHP_EXTRA_VERSION` | `string`; the current PHP extra version.
`PHP_INT_MAX` | `int`; the maximum representable value for an integer.
`PHP_INT_MIN` | `int`; the minimum representable value for an integer.
`PHP_INT_SIZE` |  `int`; the number of bytes used to represent an integer.
`PHP_MAJOR_VERSION` | `int`; the current PHP major version
`PHP_MANDIR`  | `string`; the installation location of the manual pages.
`PHP_MAXPATHLEN` |  `int`; the maximum length of a fully qualified filename supported by this build.
`PHP_MINOR_VERSION` | `int`; the current PHP minor version.
`PHP_OS` | `string`; the current operating system.
`PHP_PREFIX`  | `string`; the value to which "--prefix" was set when configured.
`PHP_RELEASE_VERSION` | `int`; the current PHP release version.
`PHP_ROUND_HALF_DOWN` | `int`; Round halves down.
`PHP_ROUND_HALF_EVEN` | `int`; Round halves to even numbers.
`PHP_ROUND_HALF_ODD` |  `int`; Round halves to odd numbers.
`PHP_ROUND_HALF_UP` | `int`; Round halves up.
`PHP_SAPI` | `string`; the Server API for this build.
`PHP_SHLIB_SUFFIX` |  `string`; build-platform's shared library suffix.
`PHP_SYSCONFDIR` |  `string`; the PHP system configuration directory.
`PHP_VERSION` | `string`; the current PHP version in the form "major.minor.release[extra]".
`PHP_VERSION_ID` |  `int`; the current PHP version.
`PHP_ZTS` | `int`; Indicates whether the compiler was built with thread safety enabled.
`STDIN` | `resource`; File resource that maps to standard input (`php://stdin`).
`STDOUT` | `resource`; File resource that maps to standard output (`php://stdout`).
`STDERR` | `resource`; File resource that maps to standard error (`php://stderr`).
`TRUE` |  `bool`; the case-insensitive Boolean value `TRUE`.

The members of the `E_*` family have values that are powers of 2, so
they can be combined meaningfully using bitwise operators.

##User-Defined Constants

A constant may be defined inside or outside of functions, inside
a [class](14-classes.md#constants), or inside an [interface](15-interfaces.md#constants).