Constants

General

A constant is a named 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, or as a d-constant by calling the library function 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.
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, an array or a resource.

The library function defined reports if a given name (specified as a string) is defined as a constant. The library function constant returns the value of a given constant whose name is specified as a string.

Examples

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 (deprecated)
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();` 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).
`__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` 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`), 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`.
`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`.
`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`.
`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`.
`E_WARNING`	`int`; Run-time warnings (non-fatal errors). Execution of the script is not halted.
`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` 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_FLOAT_DIG`	`int`; the number of decimal digits that can be rounded into a float and back without precision loss.
`PHP_FLOAT_EPSILON`	`float`; the smallest representable positive number `x`, so that `x + 1.0 != 1.0`.
`PHP_FLOAT_MIN`	`float`; the smallest representable normalized floating point number larger than zero.
`PHP_FLOAT_MAX`	`float`; the largest representable floating point number.
`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_OS_FAMILY`	`string`; the operating system family PHP was built for. Either of ‘Windows’, ‘BSD’, ‘Darwin’, ‘Solaris’, ‘Linux’ or ‘Unknown’.
`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, or inside an interface.

PHP Language Specification

Constants

General

Context-Dependent Constants

Core Predefined Constants

User-Defined Constants