After php gets the value, two dots must be added before it can be inserted into the database? ?

Question

<?php
//表单获取值后，插入到数据库语句两边需要加 点 。才可以插入数据库中。
//反之则为什么不行？  这是涉及哪一块知识？
/*问题事例：
sql2 = "INSERT INTO test2(标题,留言) 
VALUES('".$title."','".$content."')";  */
//连接数据库、
$conn = mysqli_connect('localhost','root','123456','demo');
if(mysqli_connect_errno($conn)){
	die('连接mysql失败'.mysqli_connect_error($conn));
}
else{
	echo '<h3>mysql连接成功</h3>';	
}
$sql='CREATE TABLE test2(
id int(10) primary key auto_increment,
标题 char(20) NOT NULL,
留言 text NOT NULL);';
if(mysqli_query($conn,$sql)){
	
	echo '表创建成功';
}else{
	echo '表创建失败';
}
//获取输入值
$title = $_POST['title'];
$content = $_POST['content'];

$sql2 = "INSERT INTO test2(标题,留言) 
VALUES('".$title."','".$content."')";

if (mysqli_query($conn, $sql2)) {
 echo '插入成功';
} else {
 echo '插入失败';
}
mysqli_close($conn);
?>

Answer 1

Firstly, mysql requires quotation marks when inserting Chinese characters. Secondly, php dots are used to connect characters

Answer 2

You don’t need to add any dots, you can wrap it with {}

Answer 3

. Just a connector. . If you write this without the dot '', the content will not be parsed and remain as is