Php执行SQL语句一直失败

SQL语句
$sql="INSERT INTO message (author, subject, content, date)
values ('$author', '$subject', '$content', '$current_time')";
$result=execute_sql($link,"guestbook",$sql);

echo $sql 结果如下 直接复制到MYSQL可以成功执行
INSERT INTO message (author, subject, content, date) values ('sun', 'smile', '123455666', '2016-06-27 21:48:11')

错误提示
执行失败INSERT INTO message (author, subject, content, date) values ('sun', 'smile', '123455666', '2016-06-27 21:48:11')

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''smile', '123455666', '2016-06-27 21:48:11')' at line 2

你的sql不能换行。如果非要换行,得写
$sql = "..." +
"...";

检查下字段的类型,int类型的不用加''