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Detailed explanation of odd and even numbers and variables in PHP

Mar 28, 2018 am 09:57 AM
php variable Detailed explanation

本文主要和大家介绍了PHP中单双号与变量的相关资料,本文实例代码结合文字说明,给大家介绍的非常详细,需要的朋友参考下,希望能帮助到大家。

例子


<?php
$string = "beautiful";
$time = "winter";
$str = &#39;This is a $string $time morning!&#39;;
echo $str. "<br />";
eval("\$str = \"$str\";");
echo $str;
?>
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输出:

This is a $string $time morning!
This is a beautiful winter morning!

分析:

php里,

单引号内的变量不会被执行,

双引号内的变量会被执行,

双引号内单引号里面的变量会被执行,

但是双引号内变量加上反斜杠就可以让这个变量不被执行,成为特例,比如:


<?php
$a=5;
echo &#39;$a&#39;."\n"; // 结果输出 $a
echo "$a\n"; // 结果输出 5
echo "&#39;$a&#39;\n"; // 结果输出 &#39;5&#39;
echo "\$a=".$a; // 结果输出 $a=5 ,点是连接符
?>
eval("\$str = \"$str\";");
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一开始的 \$str 就是这个意思, 前面相当于 $str= ,就是给 $str 这个变量赋值;
后面的两个反斜杠加引号就是转义,就表示引号,所以后面的是 "$str" ,这个$str 会被执行,就是 " 'This is a $string with my $name in it.' "
这里又涉及一个双引号嵌套单引号的问题,比如sql语句
$sql = "select * from user where id = '$id' ";
这里就有双引号嵌套单引号,这里面的变量 $id 是会被执行的~
也就是说双引号嵌套单引号那么单引号里的变量也会被执行,,
或者可以简单地总结:双引号内的变量如果没有前面加反斜杠的特例,则都是要执行的;
这个eval双引号内的将要被执行的代码就是 $str=" 'This is a $string with my $name in it.' "

补充:

PHP 是一门类型松散的语言

在上面的例子中,请注意我们不必告知 PHP 变量的数据类型。
PHP 根据它的值,自动把变量转换为正确的数据类型。
在诸如 C 和 C++ 以及 Java 之类的语言中,程序员必须在使用变量之前声明它的名称和类型。
PHP 变量作用域
在 PHP 中,可以在脚本的任意位置对变量进行声明。
变量的作用域指的是变量能够被引用/使用的那部分脚本。
PHP 有三种不同的变量作用域:
local(局部)
global(全局)
static(静态)
Local 和 Global 作用域

函数之外声明的变量拥有 Global 作用域,只能在函数以外进行访问。

函数内部声明的变量拥有 LOCAL 作用域,只能在函数内部进行访问。

下面的例子测试了带有局部和全局作用域的变量:

实例


<?php
$x=5; // 全局作用域
function myTest() {
 $y=10; // 局部作用域
 echo "<p>测试函数内部的变量:</p>";
 echo "变量 x 是:$x";
 echo "<br>";
 echo "变量 y 是:$y";
}
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myTest();


echo "<p>测试函数之外的变量:</p>";
echo "变量 x 是:$x";
echo "<br>";
echo "变量 y 是:$y";
?>
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在上例中,有两个变量 $x 和 $y,以及一个函数 myTest()。$x 是全局变量,因为它是在函数之外声明的,而 $y 是局部变量,因为它是在函数内声明的。

如果我们在 myTest() 函数内部输出两个变量的值,$y 会输出在本地声明的值,但是无法 $x 的值,因为它在函数之外创建。
然后,如果在 myTest() 函数之外输出两个变量的值,那么会输出 $x 的值,但是不会输出 $y 的值,因为它是局部变量,并且在 myTest() 内部创建。

注释:您可以在不同的函数中创建名称相同的局部变量,因为局部变量只能被在其中创建它的函数识别。

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