例子 <?php $string = "beautiful"; $time = "winter"; $str = 'This is a $string $time morning!'; echo $str. ""; eval("\$str = \"$str\";"); echo $str; ?> 输出: This is a $string $time morning! This is a beautiful winter morning! 分析: php里, 单引号内的变量不会被执行, 双引号内的变量会被执行, 双引号内单引号里面的变量会被执行, 但