在php中的变量中存储一个字符串并在mysql数据库中搜索该变量？

Question

If I search for a string directly it works properly but whenever I store that string in a variable and try to search that it gives me:

bool(false)

here is my code

<?php
 $mysqli = new mysqli("localhost", "root", "password", "database");
 $roll_no='9999-SO-12';
 echo $roll_no;
 $res = $mysqli->query("SELECT name,title FROM student_data WHERE roll_no=$roll_no");

 var_dump($res);
  ?>

But whenever I do it directly It works fine for example like this

<?php
 $mysqli = new mysqli("localhost", "root", "password", "database");
 $roll_no='9999-SO-12';
 $res = $mysqli->query("SELECT name,title FROM student_data WHERE roll_no='9999-SO-12'");

 var_dump($res);
  ?>

So what am I doing wrong? What's the solution?

Answer 1

You are passing the parameter as integer though it is string. Change your query to:

$mysqli->query("SELECT name,title FROM student_data WHERE roll_no='".$roll_no."'");

Answer 2

做一些这样的改变：

$res = $mysqli->query("SELECT name,title FROM student_data WHERE roll_no='".$roll_no."'");

Answer 3

请试试这个。

 $res = $mysqli->query("SELECT name,title FROM student_data WHERE roll_no='".$roll_no."'");