PHP basic study notes (12)

php基本语法

<?<span>php
    </span><span>//</span><span>这个是php单行注释</span>
    <span>/*</span><span>
        这是多行注释
    </span><span>*/</span>
    <span>//</span><span>每条php语句使用英文分号结束&mdash;&mdash;必须使用！
    </span><span>//</span><span>php是一门区分大小写的语言&mdash;&mdash;但函数名除外
    </span><span>//</span><span>php的任何一个变量名都要使用一个$符开头。</span>
?>

变量和常量

php中，变量使用一个$符号开头。变量通常是直接赋值（此时如果是第一次赋值也就表示定义）。

常量：设定一个其值不会也不应该改变的数据标识符（数据名字），比如数学上的“pi”。

$pi = 3.14;        //这是变量

define(“PI”, 3.14);    //这是定义一个常量，名字为“PI”，值为3.14，这里也就类似赋值

常量定义的同时也就是赋值，且只是这一次性的！

常量和变量的区别：

• 常量前面没有美元符号（$）
• 常量只能用 define() 函数定义，而不能通过赋值语句赋值
• 常量可以不用理会变量范围的规则而在任何地方定义和访问
• 常量一旦定义就不能被重新定义或者取消定义
• 常量的值只能是标量（即整数，浮点数，字符串，布尔）

数据类型：

         基本类型（标量类型）：

                   布尔类型（bool，boolean）： 只有两个数据：true  false

                   字符类型（string）：可以是单引号，也可以是双引号，还可以是“定界符”格式

                   整数类型（int，integer）：

                            $v1 = 10;

                            $v2 = 012;                  //以0开始的数字表示“8进制”整数，这里相当于10进制的10

                            $v3 = 0x1F;                //以0x开始的数字表示16进制整数，这里相当于10进制的31

                   浮点类型（float，double）

                            $v4 = 12.34;

                            $v5 = 12.34e5;                   //12.34乘以10的5次方

         复合类型：

                   数组类型：

                   对象类型：

         特殊类型：

                   资源类型：表示某种“外界数据”的类型。

                   空类型（null）：只有一个数据，就是“null”

3种字符串类型的区别：

$str1 = <span>"</span><span>abc\ndefg1</span><span>"</span>;    <span>//</span><span>双引号字符串中可以识别以下转义字符：\"  \n  \r  \t  \\  \$</span>
$str2 = <span>'</span><span>a\bc\ndefg2\\</span><span>'</span>;        <span>//</span><span>单引号字符串中只能识别以下转义字符：\'  \\。其中"\\"通常只用于最后一个字符
</span><span>//</span><span>下述为定界符字符串，其中的“ABCD”是随意命名的一个符号。字符串也就使用该符号结尾。
</span><span>//</span><span>定界符字符串中可以识别以下转义字符：\n  \r  \t  \\  \$</span>
$str3 = <<<<span>ABCD
abc\nde</span><span>"</span><span>f'g3</span>
<span>这叫做定界符字符串
这个范围的任何内容都是属于str3这个字符串的内容
换行也可以直接换
还可以写html和js等等。。。比如：
</span><hr />
<script><span>
    alert(</span><span>"</span><span>dddd</span><span>"</span><span>);
</span></script><span>
ABCD;
</span><span>//</span><span>特别注意：上述一行定界符字符串的结束符只能出现该名字本身和一个分号，不能有任何其他内容，比如空格，缩进（tab符）

</span><span>//</span><span>注意：在双引号字符串和定界符字符串中，可以直接识别变量并用该该变量的值来填充其内容。
</span><span>//</span><span>如果想要不被识别，则需要使用"\$"来将该$符号进行转义</span>
    $i = <span>5</span><span>;
    echo </span><span>"</span><span>$i=</span><span>"</span> . $i ;

传值方式：

         值传递（赋值传值）：将一个变量的值拷贝一份，然后赋值给另一个变量，此时两个变量的值相等（相同），但两个变量是完全独立的没有关联的变量。

                   默认情况下，在php中，简单数据类型和数组以及空类型使用值传递。

用 Reference to pass (reference value): Copy one copy represented by one variable multiple representative and assign a value to another variable, which is equivalent to two variables to point (corresponding) one of the same address — this address is also the address. Represents the location of data. At this time, when the values ​​of the two variables are equal, it also means that the two variables actually refer to a common piece of data.

                                            By default, in php, object types and resource types are passed by reference.

However:

                                                                                                                                                                                                                                                                       to be passed by reference can also be used. The syntax is:

Variable 1 = & Variable 2;

                              Example:

                      $v1 = 10;

                                                                                                                                                                                                                         $v2

                    $v1++;

                        echo $v2;

Compare js:

In js, there is only the default value transfer method, and there is no artificial setting method

In js: The types of default value transfer are: numbers, characters, Boolean, two special types

值 The type of referenced value by default is: array, object

Operator:

l Arithmetic operators: + - * / % ++ --

The

+ sign only does "arithmetic operations" and has no double meaning.

++ --The symbol meaning and usage are exactly the same as js.
• l is written before the variable, then: add (decrement) first, and then do other operations (such as assignment)

l is written after the variable, then: do other operations (such as assignment) first, and then add (subtract) yourself

The % sign only performs the AND operation on integers. If it is not an integer, it will be automatically converted to an integer first and then the remainder will be taken.

• l Comparison operators: == != > >= < <= ===(all equal) !==(not all equal)

== usually means "fuzzy equality", === means exact equality (only if the data type and data value are the same)

For various situations of fuzzy equality, please refer to the manual: Appendix〉Type Comparison Table
• l Conditional operator: (expr1) ? (expr2) : (expr3), example:

Meaning: Evaluate and judge the expression exp1. If the judgment result is true, the result of the entire expression is exp2, otherwise the result of the entire expression is exp3

$v1 = $fs>60 ? "Pass" : "Make-up exam" ; //Then v1 has two situations based on the value of fs
Supplementary knowledge: The unary operators are: !, ++, --, ~,, the binary operator is the most commonly used operator, and the ternary operator is this one.
• l Logical operators: &&(AND) ||(OR) ! (NOT)

Logical AND &&: Only when both data are true, the result is true

Logical OR||: As long as one data is true, the result is true
Logical negation! : Take the opposite value of logical data.
• l String operator: . (i.e. English period)   .= (i.e. similar to “+=")

.= is just a "shorthand", $v1 = $v1 . "abc"; è $v1 .= "abc";

• l Bit operators: &(AND) |(OR) ~(NOT) ^(XOR) <<(left shift) >>(right shift)

Bitwise operators are all performed on the binary system of numbers.

Bitwise AND &: The result is 1 only when the numbers on the two corresponding bits are both 1
Bitwise OR|: As long as one number in the corresponding bit is 1, the result is 1
Bitwise NOT~: negation, that is, 1 becomes 0, and 0 becomes 1
Bitwise XOR^: When the numbers on the two corresponding bits are different, the result is 1
• l Assignment operator: = += -= *= /= %= .=

Operator priority issue: basically the same as js.

Data type conversion

在js中，基本没有数据类型转换的概念，实际的转换都是“自动发生”的——默认转换。

         js中有两个“类似”类型转换的函数： parseInt(….)  parseFloat(…)

php中，既有默认转换，也有强制转换。

默认转换举例：

$v1 = <span>10</span> - “<span>3</span>”;        <span>//</span><span>7；</span>
    $v2 = <span>10</span> + “<span>3</span>”;        <span>//</span><span>13;</span>
    $v3 = <span>10</span> + “3abc”;    <span>//</span><span>13;</span>
    $v4 = <span>10</span> + “<span>3</span>.5abc”;    <span>//</span><span>13.5;</span>
    $v5 = <span>10</span> + “abc3”;    <span>//</span><span>10;</span>
    $v5 = <span>10</span> + “abc”;    <span>//</span><span>10</span>
    $v5 = “<span>10</span>” + “abc”;    <span>//</span><span>10</span>
    $v5 = “10ABC” + “5abc”;    <span>//</span><span>15</span>
    $v5 = “ABC10” + “abc5”;    <span>//</span><span>0</span>
    $v5 = “ABC” + “abc”;    <span>//</span><span>0</span>

………………….凡是使用算术运算符进行运算，都会“看成”数字，如果实在转不成一个数字，也会当做0来进行计算。

强制转换：

<span>语法： (要转成的目标类型)数据；
    举例：
    $v1 </span>= (<span>int</span>) “<span>3</span>.5abc”;    <span>//</span><span>3;int类型</span>
    $v1 = (<span>float</span>) “<span>3</span>.5abc”;    <span>//</span><span>3.5;    </span><span>//</span><span>这里其实是float类型</span>
    $v1 = (<span>string</span>) <span>3</span>;        <span>//</span><span>”3”;</span>
    $v1 = (<span>bool</span>) “<span>3.5</span>”;        <span>//</span><span>true</span>

——特别推荐将一个数据转换为bool类型的时候：查手册：附录〉类型比较表〉对变量 $x 进行比较 〉 if（$x）列

以上就介绍了php基础学习笔记（12），包括了方面的内容，希望对PHP教程有兴趣的朋友有所帮助。