PHP isset()与empty()的使用差别详解
PHP isset()与empty()的使用区别详解
PHP的isset()函数 一般用来检测变量是否设置
格式：bool isset ( mixed var [, mixed var [, ...]] )
功能：检测变量是否设置
返回值：
若变量不存在则返回 FALSE
若变量存在且其值为NULL，也返回 FALSE
若变量存在且值不为NULL，则返回 TURE
同时检查多个变量时，每个单项都符合上一条要求时才返回 TRUE，否则结果为 FALSE
版本：PHP 3, PHP 4, PHP 5
更多说明：
使用 unset() 释放变量之后，它将不再是 isset()。
PHP函数isset()只能用于变量，传递任何其它参数都将造成解析错误。
检测常量是否已设置可使用 defined() 函数。
PHP的empty()函数 判断值为否为空
格式：bool empty ( mixed var )
功能：检查一个变量是否为空
返回值：
若变量不存在则返回 TRUE
若变量存在且其值为""、0、"0"、NULL、、FALSE、array()、var $var; 以及没有任何属性的对象，则返回 TURE
若变量存在且值不为""、0、"0"、NULL、、FALSE、array()、var $var; 以及没有任何属性的对象，则返回 FALSE
版本：PHP 3, PHP 4, PHP 5
更多说明：
empty()的返回值=!(boolean) var，但不会因为变量未定义而产生警告信息。参见转换为布尔值获取更多信息。
empty() 只能用于变量，传递任何其它参数都将造成Paser error而终止运行。
检测常量是否已设置可使用 defined() 函数。
例子： empty() 与 isset() 的一个简单比较
<span style="color: #000000;">php </span><span style="color: #800080;">$var</span> = 0<span style="color: #000000;">; </span><span style="color: #008000;">//</span><span style="color: #008000;"> 结果为 true，因为 $var 为空 </span><span style="color: #0000ff;">if</span> (<span style="color: #0000ff;">empty</span>(<span style="color: #800080;">$var</span><span style="color: #000000;">)) { </span><span style="color: #0000ff;">echo</span> '$var is either 0 or not set at all'<span style="color: #000000;">; } </span><span style="color: #008000;">//</span><span style="color: #008000;"> 结果为 false，因为 $var 已设置 </span><span style="color: #0000ff;">if</span> (!<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$var</span><span style="color: #000000;">)) { </span><span style="color: #0000ff;">echo</span> '$var is not set at all'<span style="color: #000000;">; } </span>?> 注: 由于这是一个语言结构而非函数，因此它无法被变量函数调用。
注: empty() 只检测变量，检测任何非变量的东西都将导致解析错误。换句话说，后边的语句将不会起作用： empty(addslashes($name))。
下面是测试过的一段isset与empty函数详细例子的代码,看完这个基本上就差不多了:
<span style="color: #000000;">php </span><span style="color: #008080;">error_reporting</span>(<span style="color: #ff00ff;">E_ALL</span><span style="color: #000000;">); </span><span style="color: #0000ff;">echo</span> '<b>未定义$var</b><br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> "isset测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">isset</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )) { </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;"> ; } </span><span style="color: #0000ff;">echo</span> "empty测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">empty</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )){ </span><span style="color: #0000ff;">echo</span> '变量$var的值为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var的值不为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> "变量直接测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> ){ </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var不存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> '----------------------------------<br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> '<b>$var = \'\'</b><br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> "isset测试:<br>"<span style="color: #000000;">; </span><span style="color: #800080;">$var</span> = ''<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">isset</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )) { </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;"> ; } </span><span style="color: #0000ff;">echo</span> "empty测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">empty</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )){ </span><span style="color: #0000ff;">echo</span> '变量$var的值为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var的值不为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> "变量直接测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> ){ </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var不存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> '----------------------------------<br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> '<b>$var = 0</b><br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> 'isset测试:<br>'<span style="color: #000000;">; </span><span style="color: #800080;">$var</span> = 0<span style="color: #000000;"> ; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">isset</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )) { </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;"> ; } </span><span style="color: #0000ff;">echo</span> "empty测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">empty</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )){ </span><span style="color: #0000ff;">echo</span> '变量$var的值为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var的值不为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> "变量直接测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> ){ </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var不存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> '----------------------------------<br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> '<b>$var = null</b><br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> 'isset测试:<br>'<span style="color: #000000;">; </span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">null</span><span style="color: #000000;"> ; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">isset</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )) { </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;"> ; } </span><span style="color: #0000ff;">echo</span> "empty测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">empty</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )){ </span><span style="color: #0000ff;">echo</span> '变量$var的值为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var的值不为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> "变量直接测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> ){ </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var不存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> '----------------------------------<br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> '<b>$var ="php"</b><br>'<span style="color: #000000;">; </span><span style="color: #0000ff;">echo</span> 'isset测试:<br>'<span style="color: #000000;">; </span><span style="color: #800080;">$var</span> = "php"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">isset</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )) { </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;"> ; } </span><span style="color: #0000ff;">echo</span> "empty测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #0000ff;">empty</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> )){ </span><span style="color: #0000ff;">echo</span> '变量$var的值为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var的值不为空<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">echo</span> "变量直接测试:<br>"<span style="color: #000000;">; </span><span style="color: #0000ff;">if</span> ( <span style="color: #800080;">$var</span><span style="color: #000000;"> ){ </span><span style="color: #0000ff;">echo</span> '变量$var存在!<br>'<span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">echo</span> '变量$var不存在!<br>'<span style="color: #000000;">; } </span>?>在使用 php 编写页面程序时，我经常使用变量处理函数判断 php 页面尾部参数的某个变量值是否为空，开始的时候我习惯了使用 empty() 函数，却发现了一些问题，因此改用 isset() 函数，问题不再。
顾名思义，empty() 判断一个变量是否为“空”，isset() 判断一个变量是否已经设置。正是这种所谓的“顾名思义”，令我开始时走了些弯路：当一个变量值等于0时，empty()也会成立（True），因而会发生 一些意外。原来，empty() 和 isset() 虽然都是变量处理函数，它们都用来判断变量是否已经配置，它们却是有一定的区别：empty还会检测变量是否为空、为零。当一个变量值为0，empty() 认为这个变量同等于空，即相当于没有设置。
比如检测 $id 变量，当 $id=0 时，用empty() 和 isset() 来检测变量 $id 是否已经配置，两都将返回不同的值—— empty() 认为没有配置，isset() 能够取得 $id 的值：
<span style="color: #800080;">$id</span>=0<span style="color: #000000;">; </span><span style="color: #0000ff;">empty</span>(<span style="color: #800080;">$id</span>)?<span style="color: #0000ff;">print</span> "It's empty .":<span style="color: #0000ff;">print</span> "It's <span style="color: #800080;">$id</span> ."<span style="color: #000000;">; </span><span style="color: #008000;">//</span><span style="color: #008000;">结果：It's empty . </span><span style="color: #0000ff;">print</span> "<br>"<span style="color: #000000;">; </span>!<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$id</span>)?<span style="color: #0000ff;">print</span> "It's empty .":<span style="color: #0000ff;">print</span> "It's <span style="color: #800080;">$id</span> ."<span style="color: #000000;">; </span><span style="color: #008000;">//</span><span style="color: #008000;">结果：It's 0 . </span>
这意味着，我们在使用变量处理函数时，当该变量可能出现0的值，使用 empty() 要小心，这个时候用 isset 取代它更明智一些。
当一个php页面的 URL 尾部参数出现 id=0 时（比如：test.php?id=0），试比较：
<span style="color: #0000ff;">if</span>(<span style="color: #0000ff;">empty</span>(<span style="color: #800080;">$id</span>)) <span style="color: #800080;">$id</span>=1; - 若 id=0<span style="color: #000000;"> ，id 也会为1 </span><span style="color: #0000ff;">if</span>(!<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$id</span>)) <span style="color: #800080;">$id</span>=1; - 若 id=0 ，id 不会为1
可分开运行以下代码检测上述推断：
<span style="color: #0000ff;">if</span>(<span style="color: #0000ff;">empty</span>(<span style="color: #800080;">$id</span>)) <span style="color: #800080;">$id</span>=1<span style="color: #000000;">; </span><span style="color: #0000ff;">print</span> <span style="color: #800080;">$id</span>; <span style="color: #008000;">//</span><span style="color: #008000;"> 得到 1 </span><span style="color: #0000ff;">if</span>(!<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$id</span>)) <span style="color: #800080;">$id</span>=1<span style="color: #000000;">; </span><span style="color: #0000ff;">print</span> <span style="color: #800080;">$id</span>; <span style="color: #008000;">//</span><span style="color: #008000;">得到 0 </span>
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（”?”）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
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