PHP中的字符串中的大括号

Question

{ }PHP中字符串文字中(花括号)的含义是什么？

Answer 1

这是字符串插值的复杂(卷曲)语法.从手册:

复杂(卷曲)语法

这不称为复杂,因为语法很复杂,但因为它允许使用复杂的表达式.

可以通过此语法包含具有字符串表示的任何标量变量,数组元素或对象属性.只需像在字符串外部一样编写表达式,然后将其包装在{和中}.由于{无法进行转义,因此只有在$紧随其后才能识别此语法{.使用 {\$得到文字{$.一些例子说清楚:

<?php
// Show all errors
error_reporting(E_ALL);

$great = 'fantastic';

// Won't work, outputs: This is { fantastic}
echo "This is { $great}";

// Works, outputs: This is fantastic
echo "This is {$great}";
echo "This is ${great}";

// Works
echo "This square is {$square->width}00 centimeters broad."; 


// Works, quoted keys only work using the curly brace syntax
echo "This works: {$arr['key']}";


// Works
echo "This works: {$arr[4][3]}";

// This is wrong for the same reason as $foo[bar] is wrong  outside a string.
// In other words, it will still work, but only because PHP first looks for a
// constant named foo; an error of level E_NOTICE (undefined constant) will be
// thrown.
echo "This is wrong: {$arr[foo][3]}"; 

// Works. When using multi-dimensional arrays, always use braces around arrays
// when inside of strings
echo "This works: {$arr['foo'][3]}";

// Works.
echo "This works: " . $arr['foo'][3];

echo "This works too: {$obj->values[3]->name}";

echo "This is the value of the var named $name: {${$name}}";

echo "This is the value of the var named by the return value of getName(): {${getName()}}";

echo "This is the value of the var named by the return value of \$object->getName(): {${$object->getName()}}";

// Won't work, outputs: This is the return value of getName(): {getName()}
echo "This is the return value of getName(): {getName()}";
?>

通常,这种语法是不必要的.例如,这个:

$a = 'abcd';
$out = "$a $a"; // "abcd abcd";

表现完全相同:

$out = "{$a} {$a}"; // same

所以花括号是不必要的.但是这个:

$out = "$aefgh";

将根据您的错误级别,无法工作或产生错误,因为没有命名的变量$aefgh,所以您需要这样做:

$out = "${a}efgh"; // or
$out = "{$a}efgh";

Answer 2

至于我,花括号用作连接的替代,它们更快地键入并且代码看起来更清晰.请记住使用双引号(""),因为它们的内容由PHP 解析,因为在单引号('')中,您将获得提供的变量的文字名称:

<?php

 $a = '12345';

// This works:
 echo "qwe{$a}rty"; // qwe12345rty, using braces
 echo "qwe" . $a . "rty"; // qwe12345rty, concatenation used

// Does not work:
 echo 'qwe{$a}rty'; // qwe{$a}rty, single quotes are not parsed
 echo "qwe$arty"; // qwe, because $a became $arty, which is undefined

?>

Answer 3

例:

$number = 4;
print "You have the {$number}th edition book";
//output: "You have the 4th edition book";

没有花括号,PHP会尝试找到一个名为的变量$numberth,它不存在!