06 常量

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2023-12-01

概述

常量是一个简单值的标识符(名字)。定义之后，脚本之行期间常量的值不能改变。

常量有俩种定义方法：使用const关键字在类之外定义常量，或者使用define函数。这俩种方法略有不同，如下：

使用const关键字定义的名称必须遵循语法分析器对一个命名的规则。函数 define 定义的常量可以是任何源字符。
const关键字定义的常量是大小写敏感的。函数define的第三个参数可以设置定义的常量是否大小写敏感。
用函数define定义常量成功时反悔TRUE，否则返回FALSE。

常量只能包含标量类型、数组或者资源类型。

函数defined判断一个名称（字符串）是否已经被定义成一个常量。函数constant能够返回一个常量的值。

实例

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

上下文相关的常量

以下的常量－被称为魔法常量－在PHP脚本的全局都有效，他们的值不是确定的，而且大小写敏感：

常量名称	描述
`__CLASS__`	`string`; 当前类的名称。在 trait 方法内部, 这个值是 trait 被使用的类的名称.如果当前的命名空间就是默认值，这个值就是命名空间前加一个反斜线`\`。如果不在任何一个类的内部，这个值就是一个空字符串。
`__COMPILER_HALT_OFFSET__`	`int`; 当 `__halt_compiler();` 使用时, this constant contains the byte offset in the source file immediately following the `__halt_compiler();` token in this file.
`__DIR__`	`string`; 文件所在目录. 根目录的话，该值仅有一个文件分隔符。
`__FILE__`	`string`; 文件完整的路径和文件名。
`__FUNCTION__`	`string`; 在函数内部，该值就是声明该函数的定义名称。或许还有前缀: 如果存在命名空间，就在命名空间后加上`\`。如果不在任何函数内部，这个值就是空字符串。对于一个方法，没有父类前缀就是当前方法。（参考`__METHOD__`和匿名函数)）。
`__LINE__`	`int`; 文件当前行号。
`__METHOD__`	`string`; 在一个类的方法内部，返回当前的方法名称。而且会带着以下的前缀表示：如果存在命名空间，前缀就是命名空间后面带一个`\`；父类或者trait名带一个`::`。如果不在任何方法内部，返回结果同`__FUNCTION__`。
`__NAMESPACE__`	`string`; 当前命名空间的名称。默认命名空间下，是一个空字符串。
`__TRAIT__`	`string`; 当前 Trait 的名称。在trait的方法里，返回的夜市当前的trait名称。如果不在任何trait 里，返回空字符串。

以 __ 为前缀的常量都是预留给将来引擎的。

核心预定义常量

以下所有常量在所有文件中都已经被自动引用，均可直接使用。除了NULL、TRUE 和 FALSE，都是大小写敏感的。

常量名称	描述
`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.
`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`.
`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_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.

用户定义的常量

在函数内或函数外定义的常量，在类内部定义，或者在接口内部定义。