第一章 三/3节 PHP的语法与数据类型
第一章 3/3节 PHP的语法与数据类型
1. 语句分两种：结构定义语句和功能执行语句。
1)结构定义语句，(大括弧)。语句后面一定不要加分号：
if()
{
}
while()
{
}
for(;;)
{
}
?>
2)功能执行语句，一定要加上分号：
$a = 1.0;
$b = 1.0;
echo “aaaaaa”;
?>
3) 特殊情况下，离?>标记最近的分号可以不加分号：
echo “aaaaaaa”;
echo “bbbbbbbb”
?>
2. 注释与作用：
1）单行注释
// echo “aaaaaaaaaaaaaaa”;
2）多行注释
/*
echo “bbbbbbbbbbbb”;
*/
3) 文本注释
/**
*/
4) 注释的作用：
a. 写过的程序不用急着删除，先注释掉
b. 调试程序
c. 写帮助文档
3. 变量、常量
php是弱类型语言（相对于java,c++而言），理由如下：
$a=100;
$a=true;
echo $a;
结果是true
1）变量以$符号开头
变量是区分大小写的，其他不区分，如$a=100; $A=200;是两个变量
2）命名规则
不要是数字开头、空格、小数点、系统关键字，如下为错误的表达形式：
$name.age、$var、$4、$.12、 $a b
3）弱类型命名标准，可以赋值任意类型，变量有8种原始类型
$a=后面可以赋任意类型
四种标量类型：
――布尔型
――整型
――浮点型
――字符串
两种复合类型
――数组 $a=array(1,3,4);
――对象 class P{var $name;} $a=new P();
两种特殊类型
――资源 $a=fopen(“test.txt”, “w”); var_dump($a);结果为resource
――null
获取类型函数：
echo getType($a);
var_dump($a);// 可以看到类型与数据
4）常量，前面没有$符。
a. 只适合四种标量数据(boolean、integer、float、string)
b. 值只能在声明时赋值，并不能在运行时改变。
define(“NAME”, “testName”);
echo NAME;
5）变量与常量区别：
a. $符，常量没有，变量有。
b. 常量通过define函数定义，不能通过赋值语句赋值。
c. 常量不用遵循变量范围的规则，可以在任意地方定义和访问。
d. 常量一旦定义，不能重新定义或取消。
e. 常量值只能是标量
6）. 检查常量是否存在，用echo defined(“NAME”);输出是1表示常量NAME存在。
if(defined(“NAME”))
{
}else{
}
检查变量是否存在，isset($a);
7）PHP中常见常量
echo __FILE__;// 物理路径
echo __LINE__;// 行号
echo _PHP_VERSION;// 版本号
echo PHP_OS;// 操作系统
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