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HomeBackend DevelopmentPHP Tutorial一般何种情况上定义常量

一般何种情况下定义常量?

新手提问,
常量和变量的值一样,也是存在内存中也是存在栈区吗?
为什么会有常量这种东西呢?反正都是存值,直接用变量代替它不久好了。

常量和变量运用的典型场合有哪些


------解决方案--------------------
看名字就应该知道一些,一个是可以修改的,一个不能修改的
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关键在于 常量是全局的
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本帖最后由 xuzuning 于 2012-12-13 13:39:17 编辑
引用:
版主 变量也可以是全局的呀
是吗?你用用看呀
$a = 123;<br />define('A', 123);<br /><br />function func1() {<br />  echo $a; //Notice: Undefined variable: a<br />}<br />function func2() {<br />  echo A; //123<br />}<br /><br />func1();<br />func2();

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引用:
本帖最后由 xuzuning 于 2012-12-13 13:39:17 编辑
            引用:版主 变量也可以是全局的呀是吗?你用用看呀PHP code?123456789101112$a = 123;define('A', 123); function func1() {  echo $a; //Not……

他的意思可能是声明成 global
------解决方案--------------------
凑一下热闹
<br />$GLOBALS['a']='aa';<br />function abc(){<br />echo $GLOBALS['a'];<br />}<br />abc();<br />

------解决方案--------------------
繼續打醬油
<br />Define('D','DD');<br />$b='bbb';<br />$GLOBALS['a']='aa';<br />function abc(){<br />	Global $b;//如果不加入这一行,下边的行一定出错,这就是变量的作用范围了.<br />	echo $b;<br />	echo $GLOBALS['a'];<br />	Echo D;<br /><br />	$c='ccc';<br />}<br />abc();<br />Echo D;<br />Echo $c;//這行一定出錯,因為$c聲明的時候作用範圍是在function內

------解决方案--------------------
常量的结构定义:
typedef struct _zend_constant {<br />	zval value;//常量的值<br />	int flags; //CONST_PERSISTENT(跨进程持续存在) <br><font color='#FF8000'>------解决方案--------------------</font><br> CONST_CS(大小写敏感) <br><font color='#FF8000'>------解决方案--------------------</font><br> CONST_CT_SUBST(允许编译时被替代)<br />	char *name;//常量名<br />	uint name_len;//名称长度<br />	int module_number;//常量模块号,由用户定义的常量,此属性会被初始化为PHP_USER_CONSTANT<br />} zend_constant;




变量的结构定义
struct _zval_struct {<br />	/* Variable information */<br />	zvalue_value value;		//变量值<br />	zend_uint refcount__gc;   //引用计数,gc会根据此值来决定是否回收内存<br />	zend_uchar type;	//变量类型<br />	zend_uchar is_ref__gc; //是否是一个引用<br />};



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