PHP comparison operations and logical operations, php operation logic

PHP comparison operations and logical operations, PHP operation logic

1. The following values are judged to be true using empty():

UnassignedVariable, undeclared variable, 0, "0", "", false, null, empty array array(), object's magic method __get() Returned value

In versions lower than PHP5.0, objects without any attributes are also judged as empty by empty

Note: empty() only accepts variables or index values or attribute values of variables. Constants cannot be passed in directly, nor operation expressions can be passed in. Expressions are supported after PHP 5.5

2. Values judged as false by isset(): unassigned variables, undeclared variables, null, values returned by __get() accept the same participation as empty(), and cannot be constants and expressions.

3. Comparison of different types of data

If one of them is of boolean type or null, convert it to boolean for comparison,

Otherwise, if one is a number, convert it to number and compare ,

Otherwise, if one of them is a string, convert it to string and compare

Object type is always greater than array type and scalar type, array type is always greater than scalar type

Note these comparison results:

PHP comparison operations and logical operations, php operation logic_PHP tutorial

       <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>

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4. Type conversion rules

The value judged as true by empty() is converted to boolean type to get false , otherwise, it is true (the value returned by __get() needs to be a specific value Judgment)

The value judged as true by empty() is converted to number and the result is 0. The non-empty array is converted to number and the value is 1 (the value returned by __get() needs to be judged according to the specific value)

<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>

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Several string to number conversion scenarios:

<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>

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When converting string to number, intercept the numeric string on the left for conversion. If there is no string, return 0.

Convert other data to string:

<span>//几个转字符串的值</span>

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(<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>

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Arrays can be directly concatenated with strings but cannot be subjected to mathematical operations.

Converting the object type to boolean is always true. The object type cannot be converted to number and string, so string splicing and mathematical operations cannot be performed

The way to convert a scalar into an array is to set the first element of the array to a scalar and return the array.

The scalar is converted to object to get an instance of the stdClass class. The value of the scalar is assigned to the attribute named scalar: Object( [scalar] => 234)

Convert array to object to get an instance of stdClass class. The key of the array is the property name of the class.

Object to array conversion is a bit complicated:

Methods, static properties, and class constants are discarded

The protected attribute name is preceded by a "*"

Private attributes are prefixed with the class name (the case is exactly the same as the class name)

　Add space characters before and after these prefixes For example, an array converted from object is:

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

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The original objects include:

Public attribute a2, protected attribute v, p, which class these attributes come from cannot be identified (if overridden, the attributes of the subclass will be taken)

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>

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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

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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>

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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>

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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>
}

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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>

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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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