PHP语法$ {“field”}
这个问题在这里已经有了答案：
它被称为Complex (curly) syntax
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}中。 由于{不能被转义，这个语法只有在$紧跟在{之后才能被识别。 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()}";
?>