常量前面没有美元符号（$）；
常量只能用 define() 函数定义，而不能通过赋值语句；
常量可以不用理会变量的作用域而在任何地方定义和访问；
常量一旦定义就不能被重新定义或者取消定义；
常量的值只能是标量。

if (...) {const FOO = 'BAR';    // invalid
}
// but
if (...) {define('FOO', 'BAR'); // valid
}

const BIT_5 = 1 << 5;    // valid since PHP 5.6, invalid previously
define('BIT_5', 1 << 5); // always valid

for ($i = 0; $i < 32; ++$i) {define('BIT_' . $i, 1 << $i);
}

define('FOO', 'BAR', true);
echo FOO; // BAR
echo foo; // BAR

namespace A\B\C;
// To define the constant A\B\C\FOO:
const FOO = 'BAR';
define('A\B\C\FOO', 'BAR');

const FOO = [1, 2, 3];    // valid in PHP 5.6
define('FOO', [1, 2, 3]); // invalid in PHP 5.6, valid in PHP 7.0

class Foo {const BAR = 2; // valid
}
// but
class Baz {define('QUX', 2); // invalid
}

<?php
// 来看看这两种方法如何处理 namespaces
namespace MiddleEarth\Creatures\Dwarves;
const GIMLI_ID = 1;
define('MiddleEarth\Creatures\Elves\LEGOLAS_ID', 2);echo(\MiddleEarth\Creatures\Dwarves\GIMLI_ID);  // 1
echo(\MiddleEarth\Creatures\Elves\LEGOLAS_ID);  // 2; 注意：我们使用了 define()// Now let's declare some bit-shifted constants representing ways to enter Mordor.
define('TRANSPORT_METHOD_SNEAKING', 1 << 0); // OK!
const TRANSPORT_METHOD_WALKING = 1 << 1; //Compile error! const can't use expressions as values// 接下来, 条件常量。
define('HOBBITS_FRODO_ID', 1);if($isGoingToMordor){define('TRANSPORT_METHOD', TRANSPORT_METHOD_SNEAKING); // OK!const PARTY_LEADER_ID = HOBBITS_FRODO_ID // 编译错误: const 不能用于 if 块中
}// 最后, 类常量
class OneRing{const MELTING_POINT_DEGREES = 1000000; // OK!define('SHOW_ELVISH_DEGREES', 200); // 编译错误: 在类内不能使用 define()
}
?>

/* {{{ proto bool define(string constant_name, mixed value, boolean case_insensitive=false)Define a new constant */
ZEND_FUNCTION(define)
{char *name;int name_len;zval *val;zval *val_free = NULL;zend_bool non_cs = 0;int case_sensitive = CONST_CS;zend_constant c;if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "sz|b", &name, &name_len, &val, &non_cs) == FAILURE) {return;}if(non_cs) {case_sensitive = 0;}/* class constant, check if there is name and make sure class is valid & exists */if (zend_memnstr(name, "::", sizeof("::") - 1, name + name_len)) {zend_error(E_WARNING, "Class constants cannot be defined or redefined");RETURN_FALSE;}repeat:switch (Z_TYPE_P(val)) {case IS_LONG:case IS_DOUBLE:case IS_STRING:case IS_BOOL:case IS_RESOURCE:case IS_NULL:break;case IS_OBJECT:if (!val_free) {if (Z_OBJ_HT_P(val)->get) {val_free = val = Z_OBJ_HT_P(val)->get(val TSRMLS_CC);goto repeat;} else if (Z_OBJ_HT_P(val)->cast_object) {ALLOC_INIT_ZVAL(val_free);if (Z_OBJ_HT_P(val)->cast_object(val, val_free, IS_STRING TSRMLS_CC) == SUCCESS) {val = val_free;break;}}}/* no break */default:zend_error(E_WARNING,"Constants may only evaluate to scalar values");if (val_free) {zval_ptr_dtor(&val_free);}RETURN_FALSE;}c.value = *val;zval_copy_ctor(&c.value);if (val_free) {zval_ptr_dtor(&val_free);}c.flags = case_sensitive; /* non persistent */c.name = IS_INTERNED(name) ? name : zend_strndup(name, name_len);if(c.name == NULL) {RETURN_FALSE;}c.name_len = name_len+1;c.module_number = PHP_USER_CONSTANT;if (zend_register_constant(&c TSRMLS_CC) == SUCCESS) {RETURN_TRUE;} else {RETURN_FALSE;}
}
/* }}} *//* {{{ proto bool defined(string constant_name)Check whether a constant exists */
ZEND_FUNCTION(defined)
{char *name;int name_len;zval c;if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &name, &name_len) == FAILURE) {return;}if (zend_get_constant_ex(name, name_len, &c, NULL, ZEND_FETCH_CLASS_SILENT TSRMLS_CC)) {zval_dtor(&c);RETURN_TRUE;} else {RETURN_FALSE;}
}
/* }}} */

ZEND_API int zend_register_constant(zend_constant *c TSRMLS_DC)
{char *lowercase_name = NULL;char *name;int ret = SUCCESS;ulong chash = 0;#if 0printf("Registering constant for module %d\n", c->module_number);
#endifif (!(c->flags & CONST_CS)) {/* keep in mind that c->name_len already contains the '\0' */lowercase_name = estrndup(c->name, c->name_len-1);zend_str_tolower(lowercase_name, c->name_len-1);lowercase_name = (char*)zend_new_interned_string(lowercase_name, c->name_len, 1 TSRMLS_CC);name = lowercase_name;chash = IS_INTERNED(lowercase_name) ? INTERNED_HASH(lowercase_name) : 0;} else {char *slash = strrchr(c->name, '\\');if(slash) {lowercase_name = estrndup(c->name, c->name_len-1);zend_str_tolower(lowercase_name, slash-c->name);lowercase_name = (char*)zend_new_interned_string(lowercase_name, c->name_len, 1 TSRMLS_CC);name = lowercase_name;chash = IS_INTERNED(lowercase_name) ? INTERNED_HASH(lowercase_name) : 0;} else {name = c->name;}}if (chash == 0) {chash = zend_hash_func(name, c->name_len);}/* Check if the user is trying to define the internal pseudo constant name __COMPILER_HALT_OFFSET__ */if ((c->name_len == sizeof("__COMPILER_HALT_OFFSET__")&& !memcmp(name, "__COMPILER_HALT_OFFSET__", sizeof("__COMPILER_HALT_OFFSET__")-1))|| zend_hash_quick_add(EG(zend_constants), name, c->name_len, chash, (void *) c, sizeof(zend_constant), NULL)==FAILURE) {/* The internal __COMPILER_HALT_OFFSET__ is prefixed by NULL byte */if (c->name[0] == '\0' && c->name_len > sizeof("\0__COMPILER_HALT_OFFSET__")&& memcmp(name, "\0__COMPILER_HALT_OFFSET__", sizeof("\0__COMPILER_HALT_OFFSET__")) == 0) {name++;}zend_error(E_NOTICE,"Constant %s already defined", name);str_free(c->name);if (!(c->flags & CONST_PERSISTENT)) {zval_dtor(&c->value);}ret = FAILURE;}if (lowercase_name && !IS_INTERNED(lowercase_name)) {efree(lowercase_name);}return ret;
}