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empty 和 isset的区别和联系

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Jun 23, 2016 pm 02:34 PM

PHP 是一门弱类型的语言, 和 C# / JAVA 不一样, 在声明一个变量的时候, 无需声明它的类型. 因此, 可能会出现很多特殊的值, 偶尔会出现一些奇奇怪怪的问题.

下午, 从国外网站偷来一个真值表, 嘿嘿, 很直观, 很详细. 不错~ 赶紧发上来.

empty 和 isset的区别和联系

要说它们的联系，其共同点就是empty()和isset()都是变量处理函数，作用是判断变量是否已经配置，正是由于它们在处理变量过程中有很大的相似性，才导致对它们的关系认识不足。单从empty()和isset()这两个函数本身来考虑的话会把人弄得更糊涂，换一个角度来它。empty()和isset()的处理对象无外乎未定义变量，0，空字符串。
如果变量为0，则empty()会返回TRUE，isset()会返回TRUE；

如果变量为空字符串，则empty()会返回TRUE，isset()会返回TRUE；

如果变量未定义，则empty()会返回TRUE，isset()会返回FLASE；

手册中对empty()的解释如下：
描述bool empty( mixed var )
如果 var 是非空或非零的值，则 empty() 返回 FALSE 。换句话说，""、0、"0"、 NULL 、 FALSE 、array()、var $var; 以及没有任何属性的对象都将被认为是空的，如果 var 为空，则返回 TRUE 。
手册中对isset()的解释如下：
isset()检测变量是否设置
描述bool isset ( mixed var [, mixed var [, ...]] )
如果 var 存在则返回 TRUE ，否则返回 FALSE 。
如果已经使用 unset() 释放了一个变量之后，它将不再是 isset() 。若使用 isset() 测试一个被设置成 NULL 的变量，将返回 FALSE 。同时要注意的是一个 NULL 字节（"\0"）并不等同于 PHP 的 NULL 常数。
警告: isset() 只能用于变量，因为传递任何其它参数都将造成解析错误。若想检测常量是否已设置，可使用 defined()函数。
当要判断一个变量是否已经声明的时候可以使用 isset 函数
当要判断一个变量是否已经赋予数据且不为空可以用 empty 函数
当要判断一个变量存在且不为空先isset 函数再用 empty 函数

转自：http://dhost.info/yeah/read.php/88.htm

http://www.cnblogs.com/JemBai/archive/2009/04/21/1440269.html

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