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What are the ways to define strings in php

爱喝马黛茶的安东尼

Aug 27, 2019 pm 04:33 PM

phpstringdefinition

What are the ways to define strings in php

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。

一、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】

例如：

<?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。

相关推荐：《PHP入门教程》

二、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、\$";

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。

单双引号之间的区别：

（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。

三、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。

例如：

<?php    
$var = &#39;this is suifeng&#39;;    
echo <<<"EOT"    
<!DOCTYPE html>    
<html>    
<head>    
<meta http-equiv="Content-Type" content="text/html; charset=utf8" />    
<title>"随风"</title>    
</head>    
<body>    
66666666666<br/>    
"666666666"<br/>    
$var<br/>    
\$var<br/>    
\n666<br>    
</body>    
</html>    
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。

四、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

例如：

<?php    
$var = &#39;this is suifeng&#39;;    
echo <<<&#39;EOT&#39;    
<!DOCTYPE html>    
<html>    
<head>    
<meta http-equiv="Content-Type" content="text/html; charset=utf8" />    
<title>"随风"</title>    
</head>    
<body>    
&#39;02020&#39;<br/>    
\&#39;<br/>    
66666666666<br/>    
"666666666"<br/>    
$var<br/>    
\$var<br/>    
\n666<br>    
</body>    
</html>    
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 '

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