问题描述
我使用 PHP 已经很长时间了，但我只是看到类似的东西，
I have used PHP for a long time, but I just saw something like,
${ }
准确地说，我在 PHP Mongo 页面中看到了这一点:
To be precise, I saw this in a PHP Mongo page:
$m = new Mongo("mongodb://${username}:${password}@host");
那么，${ } 做了什么?使用 Google 或 PHP 文档搜索 $、{ 和 } 等字符非常困难.
So, what does ${ } do? It is quite hard to search with Google or in the PHP documentation for characters like $, { and }.
推荐答案
${ }(美元符号大括号)被称为 简单语法.
${ } (dollar sign curly bracket) is known as Simple syntax.
它提供了一种嵌入变量、数组值或对象的方法以最少的努力将属性放入字符串中.
It provides a way to embed a variable, an array value, or an object property in a string with a minimum of effort.
如果遇到美元符号($)，解析器会贪婪地认为尽可能多的标记来形成一个有效的变量名.附上花括号中的变量名以明确指定结尾名字.
If a dollar sign ($) is encountered, the parser will greedily take as many tokens as possible to form a valid variable name. Enclose the variable name in curly braces to explicitly specify the end of the name.
<?php
$juice = "apple";
echo "He drank some $juice juice.".PHP_EOL;
// Invalid. "s" is a valid character for a variable name, but the variable is $juice.
echo "He drank some juice made of $juices.";
// Valid. Explicitly specify the end of the variable name by enclosing it in braces:
echo "He drank some juice made of ${juice}s.";
?>
上面的例子会输出:
He drank some apple juice.
He drank some juice made of .
He drank some juice made of apples.
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