PHP中字符串中的大括号
PHP中字符串文字中{ } （花括号）的含义是什么？
这是字符串插值的复杂（曲线）语法。 从手册：
复杂（卷曲）的语法
这不称为复杂的，因为它的语法很复杂，但是因为它允许使用复杂的表达式。
任何具有字符串表示的标量变量，数组元素或对象属性均可通过此语法包含在内。 简单地写出表达式的方式与字符串外部的方式相同，然后将其包装在{和} 。 由于{不能被转义，这个语法只有在$紧跟在{才能被识别。 使用{$获取文字{$ 。 一些例子要说清楚：
<?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";
表现与此完全相同：
$out = "{$a} {$a}"; // same
所以大括号是不必要的。 但是这个：
$out = "$aefgh";
将取决于你的错误级别，或者不工作或产生错误，因为没有名为$aefgh的变量，所以你需要这样做：
$out = "${a}efgh"; // or
$out = "{$a}efgh";
对我来说，花括号可以代替拼接，它们键入更快，代码看起来更干净。 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字面名称：
<?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
?>
例：
$number = 4;
print "You have the {$number}th edition book";
//output: "You have the 4th edition book";
如果没有大括号，PHP会尝试查找名为$numberth的变量，该变量不存在！