PHP 的比较运算与逻辑运算，php运算逻辑_PHP教程

PHP 的比较运算与逻辑运算，php运算逻辑

1、以下值用 empty() 被判断为true：

未赋值变量、未声明变量、0、"0"、""、false、null、空数组 array() 、对象的魔术方法 __get() 返回的值

在低于 PHP5.0 的版本中，没有任何属性的对象也被 empty 判断为 true

注意：empty() 只接受变量或变量的索引值或属性值，不能直接传入常量，也不能传入运算表达式，PHP 5.5 之后支持表达式

 

2、被 isset() 判断为 false 的值：未赋值变量、未声明变量、null、__get() 返回的值,接受的参与 empty()一样，不能是常量和表达式.

3、不同类型的数据比较

如果有一个是 boolean 型或者 null， 转换成 boolean 比较,

否则如果有一个是 number，转换成 number 比较，

否则如果有一个是 string，转换成 string 比较

object 类型总是大于 array  类型和标量类型，array 类型总是大于 标量类型

 

注意这些比较结果：

       <span>//</span><span>0开头的数字字符串转数字时不会按八进制转换，而是简单地丢弃把 '0' 丢弃按数字进行比较，</span>

      123=='0123' <span>//</span><span>true</span>
      "123"<"0124" <span>//</span><span>true，0开头的数字字符串直接按十进制数字比较而非八进制</span>

      "012" == 10 <span>//</span><span> false</span>
       012 == 10  <span>//</span><span> true</span>
       0x12 == 18  <span>//</span><span> true</span>
       "0x12" == 18 <span>//</span><span> true</span>
 

 <span>false</span> < <span>true</span>; <span>//</span><span>true</span>
 2><span>true</span>; <span>//</span><span> false</span>
 2==<span>true</span>; <span>//</span><span> true </span>
 <span>null</span>==0; <span>//</span><span>true</span>
-1<0;<span>//</span><span>true</span>
-1<<span>null</span>;<span>//</span><span>false ,-1 转 bool 是true</span>

 

4、类型转换规则

被 empty() 判断为 true 的值转换为 boolean 型得到 false ,反之，得到 true  ( __get() 返回的值需按具体的值判断)

被 empty() 判断为 true 的值转换成 number 得 0，非空的 array 转 number 得到1  ( __get() 返回的值需按具体的值判断)

<span>class</span><span> Test{
   </span><span>private</span> <span>$k</span>=1<span>;
   </span><span>public</span> <span>function</span> __get(<span>$propertyName</span><span>){
       </span><span>return</span> 123<span>;
   }
}

</span><span>$obj</span> = <span>new</span><span> Test();
</span><span>echo</span> json_encode(<span>empty</span>(<span>$obj</span>->k)); <span>//</span><span>true</span>
<span>echo</span> json_encode(<span>isset</span>(<span>$obj</span>->k)); <span>//</span><span>false</span>
<span>echo</span> json_encode((bool)(<span>$obj</span>->k)); <span>//</span><span>true</span>

 

几个string 转 number 的场景： 

<span>echo</span> 'abc'*1 ; <span>//</span><span>0 </span>
<span>echo</span> '012'*1; <span>//</span><span>12  乘法：可以转换十六进制数，不是数字开头则返回 0</span>
<span>echo</span> '0x12.123'*1; <span>//</span><span>18</span>

<span>echo</span> (<span>float</span>)'0x12' ;<span>//</span><span>0 </span>
<span>echo</span> (int)'0x12' ; <span>//</span><span>0 不能处理十六进制数</span>
<span>echo</span> (<span>float</span>)'12abc'; <span>//</span><span>12 截取左侧字符串</span>
<span>echo</span> (<span>float</span>)'abc'; <span>//</span><span> 0 不是数字返回0</span>

<span>is_numeric</span>('0x123'); <span>//</span><span>true 能识别十六进制数</span>
<span>is_numeric</span>('0x123.123'); <span>//</span><span>false 识别目标是整个字符串而非截取前面一部分</span>

 

 

 

string 转 number 时截取左侧的数字字符串进行转换，如果没有则返回 0.

 

 

其他数据转字符串：

<span>//几个转字符串的值</span>

(<span>string</span>)0 ; <span>//</span><span> "0"</span>
(<span>string</span>)<span>true</span>; <span>//</span><span> "1"</span>
(<span>string</span>)<span>false</span>; <span>//</span><span> ""</span>
(<span>string</span>)<span>null</span>; <span>//</span><span> ""</span>
(<span>string</span>)<span>array</span>(); <span>//</span><span> "<span>Array</span>"</span>

数组可以直接进行字符串拼接操作但不能进行数学运算.

object 类型转换成 boolean 总是 true， object 类型不能转换成 number 和 string ,因此也不能进行字符串拼接和数学运算

标量转换成 array 的方式是将数组第一个元素设置成标量，返回该数组。

标量转换成 object 得到一个 stdClass 类的实例，标量的值被赋给名为 scalar 的属性： Object(    [scalar] => 234)

array 转 object 得到一个 stdClass 类的实例，数组的 key 为实力的属性名。

object 转 array 有点复杂：

           方法、静态属性、类常量被丢弃

           保护属性名称前面被被加上一个 "*"

           私有属性前面被加上类名作为前缀（大小写与类名完全相同）

　　　　这些前缀的前后都加上空字符 \0

例如一个由 object 转换来的 array 为：

<span>Array</span>(    [*v] => 444    [bf] => 333    [bk] => 99977    [Ak] => 999    [*p] => 888    [a2] => 22)

 

原对象中有： 

           public 属性 a2， protected 属性 v、p ，这些属性来自哪个类无法鉴别（被重写则取子类的属性）

          来自类 b 的 private 属性 f、k，(从数组 key 来看，以bf为例，无法判断他是属性名为bf，还是来自类b的私有属性f)

          来自类 A 的 private 属性 k 

          无法鉴别 b 和 A 哪个是子类哪个是父类(仅从 array 的key来看，也无法推断出原对象构造自哪个类)

 

例子：

<span>class</span><span> A {
    </span><span>private</span> <span>$A</span> = 'private property, $A of class A'; <span>//</span><span> This will become '\0A\0A'</span>
    <span>protected</span> <span>$C</span> = 'protected property, $C of class A'<span>;
}

</span><span>class</span> B <span>extends</span><span> A {
    </span><span>private</span> <span>$A</span> = 'private property, $A of class B'; <span>//</span><span> This will become '\0B\0A'</span>
    <span>public</span> <span>$AA</span> = 'public property, $AA of class B'; <span>//</span><span> This will become 'AA'</span>
    <span>protected</span> <span>$B</span> = 'protected property, $B of class B'<span>;
}

</span><span>$arr</span> = (<span>array</span>) <span>new</span><span> B();

</span><span>foreach</span> (<span>$arr</span> <span>as</span> <span>$key</span> => <span>$value</span><span>) {
    </span><span>echo</span> '<br />'<span>;
    </span><span>echo</span> <span>$key</span> .',length: '.<span>strlen</span>(<span>$key</span>).' value: '.<span>$value</span><span>;
}</span>

 

输出结果：

BA,length: 4 value: <span>private</span> property, <span>$A</span> of <span>class</span><span> B
AA</span>,length: 2 value: <span>public</span> property, <span>$AA</span> of <span>class</span><span> B
</span>*B,length: 4 value: <span>protected</span> property, <span>$B</span> of <span>class</span><span> B
AA</span>,length: 4 value: <span>private</span> property, <span>$A</span> of <span>class</span><span> A
</span>*C,length: 4 value: <span>protected</span> property, <span>$C</span> of <span>class</span> A

 

 

5、 逻辑运算总是返回 true 或 false (写多了 javascript 的人要注意),逻辑运算符优先级从高到低 为 &&、 ||、 and、 or ，逻辑运算符的短路效果可以使用语句中，但记住他们不会像 javascript 中那样返回一个 不是 boolean 类型的值，在表达式中使用要注意。

<span>$a</span> = 1<span>;
</span><span>$b</span>=0<span>;
</span><span>$b</span> and <span>$a</span> = 100<span>;
</span><span>echo</span> <span>$a</span>; <span>//</span><span>1</span>
<span>$b</span> || <span>$a</span> = 200<span>;
</span><span>echo</span> <span>$a</span>; <span>//</span><span>200</span>

 

 

 

6、switch 的比较不是 "===" 而是 "==" (在 javascript 中是 "===")

 

7、 在 php4 中，object 之间的比较方式与 array 相同，在 php5 中 , object 类型间的 "==" 比较为 true的前 提是，他们属于同一个类的实例(当然还要进行属性的比较，这类似标量的"==="比较)，object 间的 "===" 比较为 true 的前提是他 们 就是同一个对象。

 

在 PHP4 中 ，不包括任何成员变量的对象 被 empty() 判断为 true

字符串偏移 offset 取字符的 empty() 判定： 取对应 offset 的字符进行判断，在PHP5.4 以前，使用索引从字符串中取字符时会先将索引进行取整，因此左侧不包含数字的字符串都被转换成0，PHP5.4之后，不再对非整形格式的字符串索引进行取整，因此判断为 true, 同理，isset() 判定为false. 如：

<span>$str</span> = 'ab0d'<span>;
</span><span>empty</span>(<span>$str</span>[0]); <span>//</span><span>false</span>
<span>empty</span>(<span>$str</span>[0.5]); <span>//</span><span>false  索引被向下取整 为 0</span>
<span>empty</span>(<span>$str</span>["0.5"]); <span>//</span><span>false 索引被向下取整 为 0，PHP5.4之后不取证，判定为 true </span>
<span>empty</span>(<span>$str</span>[2]); <span>//</span><span>true ，取得的字符为 "0"</span>
<span>empty</span>(<span>$str</span>["3"]); <span>//</span><span>false ，取得的字符为 "d"</span>
<span>empty</span>(<span>$str</span>[4]); <span>//</span><span>true ,索引超出范围,notice 警告，但 empty() 会忽略警告</span>
<span>empty</span>(<span>$str</span>['a']); <span>//</span><span> false ,左侧不包含数字字符串索引 PHP5.4之前被处理为 $str[0]，PHP5.4之后，直接为判定 true </span>

 

无论是“不等于”还是”==“ ，不要在 PHP 的跨类型数据比较中使用”传递性“：

$a == $b; //true

$b == $c; //true

并不能说明 $a == $c 为 true

 

数组的比较方法

<span>//</span><span> 数组是用标准比较运算符这样比较的</span>
<span>function</span> standard_array_compare(<span>$op1</span>, <span>$op2</span><span>)
{
    </span><span>if</span> (<span>count</span>(<span>$op1</span>) < <span>count</span>(<span>$op2</span><span>)) {
        </span><span>return</span> -1; <span>//</span><span> $op1 < $op2</span>
    } <span>elseif</span> (<span>count</span>(<span>$op1</span>) > <span>count</span>(<span>$op2</span><span>)) {
        </span><span>return</span> 1; <span>//</span><span> $op1 > $op2</span>
<span>    }
    </span><span>foreach</span> (<span>$op1</span> <span>as</span> <span>$key</span> => <span>$val</span><span>) {
        </span><span>if</span> (!<span>array_key_exists</span>(<span>$key</span>, <span>$op2</span><span>)) {
            </span><span>return</span> <span>null</span>; <span>//</span><span> uncomparable</span>
        } <span>elseif</span> (<span>$val</span> < <span>$op2</span>[<span>$key</span><span>]) {
            </span><span>return</span> -1<span>;
        } </span><span>elseif</span> (<span>$val</span> > <span>$op2</span>[<span>$key</span><span>]) {
            </span><span>return</span> 1<span>;
        }
    }
    </span><span>return</span> 0; <span>//</span><span> $op1 == $op2</span>
}

 

 

 

8、三元运算符 ?: ，跟其他大多数编程语言不一样，PHP 的三元运算符是左结合的！

    <span>$arg</span> = 'T'<span>;    
    </span><span>$vehicle</span> = ( ( <span>$arg</span> == 'B' ) ? 'bus' :<span>    
                 ( </span><span>$arg</span> == 'A' ) ? 'airplane' :<span>   
                 ( </span><span>$arg</span> == 'T' ) ? 'train' :<span>    
                 ( </span><span>$arg</span> == 'C' ) ? 'car' :<span>    
                 ( </span><span>$arg</span> == 'H' ) ? 'horse' :    
                 'feet'<span> );    
    </span><span>echo</span> <span>$vehicle</span>;   <span>//</span><span>horse</span>

 

三元运算表达式被划分为

( <span>$arg</span> == 'B' ) ? 'bus' : ( <span>$arg</span> == 'A'<span> ) 
                                    </span>? 'airplane' : ( <span>$arg</span> == 'T'<span> ) 
                                                             </span>? 'train' : ( <span>$arg</span> == 'C'<span> )
                                                                               </span>? 'car' : ( <span>$arg</span> == 'H'<span> )
                                                                                                    </span>? 'horse' : 'feet' ;   

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