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Four ways to define strings in PHP

Feb 06, 2018 pm 03:56 PM
phpstringdefinition

本文主要和大家介绍PHP定义字符串的四种方式,非常不错,具有参考借鉴价值,需要的朋友可以参考下,希望能帮助到大家。

1、简介

在PHP中这门语言中,因为是弱类型语言,因此使用变量时不需提前定义即可使用。

我们在使用php进行开发的时候,大多数使用双引号、单引号进行定义字符串。既然有这两种方式,那么他们之间肯定是有区别的。

并且,除了单双引号定义字符串这两种方式外,php还增加了heredoc和nowdoc这两种语法糖进行补充。

2、单引号与双引号

2.1、单引号

在单引号中,任何特殊字符都会按原样输出【除\\、\‘将会被转义输出】

eg.

<?php $var = &#39;this is a var!&#39;;
echo &#39;666$var666、\&#39;、\"、\\、\n、\r、\t、\$&#39;;

 

从上面例子中我们可以明显看到,变量是没有被解析的;同时带反斜杠的转义字符仅仅只有单引号、反斜杠被转义输出了,其他的是直接输出。

2.2、双引号

在双引号中,遇到$将会解析该变量;

双引号中会转义反斜杠的转义字符;

<?php $var = &#39;this is a var!&#39;; 
echo "666$var 666、\&#39;、\"、\\、\n、\r、\t、\$";
转义字符代码 转义字符的含义
\ " 双引号
\ ' 单引号
\ \ 反斜杠
\ n 换行符
\ r 回车符
\ t 制表符
\ $ 美元符号

由上面例子我们可以知道,双引号中的变量被解析了,替换成字符串;其他带反斜杠的转义字符都进行了相应的转义。

 

2.3、单双引号之间的区别

以上两者主要区别:

1、 双引号会替换变量的值,而单引号会把它当做字符串输出。

2、对于转义的支持

3、性能速度问题,由于双引号中的字符串需要检测是否含有$符号修饰的变量,因此从理论上讲,单引号是比较快的。

3、heredoc与nowdoc

3.1、heredoc

heredoc可以理解为不使用双引号来定义字符串,但效果与使用双引号一致。【此时双引号相当于普通字符】

语法要求:

1、开始标记和结束标记使用相同的字符串,通常以大写字母来写。

2、开始标记后不能出现空格或多余的字符。

3、结束标记必须顶头写,不能有缩进和空格,且在结束标记末尾要有分号 。

4、位于开始标记和结束标记之间的变量可以被正常解析,但是函数则不可以。

eg.

 <?php  $var = &#39;this is suifeng&#39;; 
 echo <<<"EOT"
 <!DOCTYPE html>
 
 
   <meta>
   <title>"随风"</title>
 
 
 66666666666<br>
 "666666666"<br>
 $var<br>
 \$var<br>
 \n666<br>
 
 
 EOT;

 

从打印的结果我们可以看出:此时双引号仅仅只是一个普通字符;其他的效果和双引号的字符串效果一致。

3.2、nowdoc

同理,nowdoc是不使用单引号来定义,但效果相当于使用单引号来定义的字符串。【此时单引号相当于普通字符】

语法同heredoc

eg.

 <?php  
 $var = &#39;this is suifeng&#39;; 
 echo <<<&#39;EOT&#39; 
 <!DOCTYPE html>
 
 
   <meta>
   <title>"随风"</title>
 
 
 '02020'<br>
 \'<br>
 66666666666<br>
 "666666666"<br>
 $var<br>
 \$var<br>
 \n666<br>
 
 
 EOT;

 

注意:

定界符之后必须不能有任何字符【特别是空格字符有时会没注意到】,否则将会出错。

如上例中的EOT后面有空格就会报这个错误:【 Parse error : syntax error, unexpected '

相关推荐:

php截取字符串函数,php自定义字符串截取方法

PHP 自定义字符串中的变量名解析

php中单引号与双引号定义字符串的不同

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