PHP 语法 变量
您无法在浏览器中通过查看源文档的方式来查看 PHP 的源代码 - 您只能看到 PHP 文件的输出，即纯粹的 HTML。这是因为在结果返回浏览器之前，脚本就已经在服务器执行了。
基本的 PHP 语法PHP 的脚本块以 结束。您可以把 PHP 的脚本块放置在文档中的任何位置。
当然，在支持简写的服务器上，您可以使用 和 ?> 来开始和结束脚本块。
不过，为了达到最好的兼容性，我们推荐您使用标准形式 (
<?php?>
PHP 文件通常会包含 HTML 标签，就像一个 HTML 文件，以及一些 PHP 脚本代码。
在下面，我们提供了一段简单的 PHP 脚本，它可以向浏览器输出文本 "Hello World"：
<html><body><?phpecho "Hello World";?></body></html>
PHP 中的每个代码行都必须以分号结束。分号是一种分隔符，用于把指令集区分开来。
有两种通过 PHP 来输出文本的基础指令：echo 和 print。在上面的例子中，我们使用了 echo 语句来输出文本 "Hello World"。
PHP 中的注释在 PHP 中，我们使用 // 来编写单行注释，或者使用 /* 和 */ 来编写大的注释块。
<html><body><?php//This is a comment/*This isa commentblock*/?></body></html>
变量用于存储值，比如数字、字符串或函数的结果，这样我们就可以在脚本中多次使用它们了。
PHP 中的变量
变量用于存储值，比如数字、文本字符串或数组。
一旦设置了某个变量，我们就可以在脚本中重复地使用它。
PHP 中的所有变量都是以 $ 符号开始的。
在 PHP 中设置变量的正确方法是：
$var_name = value;
PHP 的入门者往往会忘记在变量的前面的 $ 符号。如果那样做的话，变量将是无效的。
让我们试着创建一个存有字符串的变量，和一个存有数值的变量：
<?php$txt = "Hello World!";$number = 16;?>
PHP 是一门松散类型的语言（Loosely Typed Language）
在 PHP 中，不需要在设置变量之前声明该变量。
在上面的例子中，您看到了，不必向 PHP 声明该变量的数据类型。
根据变量被设置的方式，PHP 会自动地把变量转换为正确的数据类型。
在强类型的编程语言中，您必须在使用前声明变量的类型和名称。
在 PHP 中，变量会在使用时被自动声明。
变量的命名规则 变量名必须以字母或下划线 "_" 开头。 变量名只能包含字母数字字符以及下划线。 变量名不能包含空格。如果变量名由多个单词组成，那么应该使用下划线进行分隔（比如 $my_string），或者以大写字母开头（比如 $myString）。
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