php中单引号和双引号所包围的字符串有什么区别

$name = "John";// 单引号字符串：按字面意义解释，变量名直接显示$single_quoted = 'Hello $name';echo $single_quoted; // 输出：Hello $name// 双引号字符串：变量插值，显示变量值$double_quoted = "Hello $name";echo $double_quoted; // 输出：Hello John