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Detailed explanation of PHP variables and dynamic string insertion into variables (case)

墨辰丷

May 17, 2018 am 09:25 AM

phpstringDetailed explanation

这篇文章主要介绍了PHP正在进行时-变量详解及字符串动态插入变量的方法，非常不错，具有参考借鉴价值,需要的朋友可以参考下

在PHP中，变量是$+变量名，变量名遵循标识符的命名规则，可以以字母、下划线开头，可以由数字、下划线、字母组成合法的变量名。

变量声明

所有变量在使用之前应该进行声明，而且最好带上注释，虽然在PHP中可以不显示声明变量。声明变量之后，可以为变量进行赋值；变量的赋值有两种类型值赋值和引用赋值。

<?php
  #合法的声明变量
  $_name;
  $account;
  $show_title;
  #赋值
  $color="red";
  #引用赋值
  $user_color=&$color;
?>

变量的作用域

在PHP脚本中的任何位置都可以声明变量，但是根据位置的不同，作用域的范围也不同。

局部变量

一般局部变量都是在函数中声明，即它只能在函数中被引用，函数退出时改变量及其值被销毁。如果在函数外使用该变量，PHP会把它当作另外一个变量进行处理，与函数中的变量无任何关系。

<?php
  $x=5;
  function show_x() {
    $x=3;
    echo $x;
  }
  show_x();
  echo $x;
?>

函数参数

函数参数更具其传递的方式分为两种，按值传递和按引用传递；如果是按值传递，参数的作用域只限于函数内，函数退出后参数被销毁。但按引用传递，参数的作用域范围不仅在函数内部。

<?php
  $cost_fish=20.10;
  $cost_apple=2.45;
  #引用参数前面需要加上&符号
  function rise($cost_fish,&$cost_apple) {
    $cost_fish++;
    $cost_apple++;
  }
  rise($cost_fish,$cost_apple);
  echo $cost_fish."值传递参数<--->引用传递参数".$cost_apple;
?>

全局变量

可以在脚本中的任何一个地方访问全局变量，但在函数中要修改全局变量的时候需要使用关键字global显式的声明。

<?php
  $x=5;
  function show_x() {
    global $x;
    $x++;
  }
  show_x();
?>

通过$GLOBAL数组也可以声明全局变量

<?php
  $x=5;
  function show_x() {
    global $x;
    $x++;
  }
  show_x();
?>

使用全局变量一定要谨慎，用多了之后很容易造成代码混乱。

静态变量

静态变量与函数参数不同在于，静态变量声明的时候多了static关键字，使得静态变量在函数退出之后不会被销毁，并且保留这个值在函数再次调用的时候使用该值。

<?php
  function spend() {
    static $date=0;
    $date++;
    echo $date;
  }
  spend();
  spend();
?>

PHP预定义变量

PHP中预定义了很多变量，可以在执行脚本的任何位置访问他们，这些变量主要是提供了大量关于环境的信息。

  //打印出$_SERVER 预定义的变量信息
  foreach($_SERVER as $var =>$value){
    echo "$var=>$value<br/>";
  }

先看下PHP正在进行时-字符串动态插入变量的内容。具体内容详情如下所示：

在PHP中，一般用双引号或者单引号将字符串括起来。

  echo "张三李四王五叫上赵六一起去&#39;喝酒&#39;".

如果要动态将将数据插入到字符串中，我们除了使用.号拼接，还可以通过使用{}来标识字符串中动态的部分。

<?php
  $name="赵六";
  echo "张三李四王五\n叫上$name一起去\&#39;喝酒\&#39;.";
  echo "</br>";
  echo "张三李四王五\n叫上\$name一起去\&#39;喝酒\&#39;.";
  echo "</br>";
  echo "张三李四王五\n叫上{$name}一起去&#39;喝酒&#39;.";
  echo "</br>";
  echo &#39;张三李四王五\n叫上$name一起去\&#39;喝酒\&#39;.&#39;;
  echo "</br>";
  echo &#39;张三李四王五\n叫上{$name}一起去\&#39;喝酒\&#39;.&#39;;
  echo "</br>";
?>

通过上面的代码我们发现：

在双引号中直接插入变量是无效的；
在双引号中可以通过反斜杠()转义字符，但双引号中的单引号不需要通过反斜杠转义；
在单引号中除了可以转义单引号，对其他的字符反斜杠转义是无效的；
通过{}可以在双引号字符串中动态插入变量。

相关推荐：

PHP中的变量 Sessions

PHP变量范围详解

php扩展开发之变量设置和销毁

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