PHP MySql错误无法从数据库读取id [重复]

Question

This question already has an answer here:

• When to use single quotes, double quotes, and backticks in MySQL 12 answers

I'm following login/register tutorial in php and mysql. As you can see in this video: http://www.youtube.com/watch?v=JUk2e8oqOvI&list=PLE134D877783367C7

You can find function user_id_from_username on 06:28 minute.

I have exact function:

function user_id_from_username($username){                  
    $username = sanitize($username);
    return mysql_result(mysql_query("SELECT 'user_id' FROM 'users' WHERE 'username' = '$username'"), 0, 'user_id');
}

But this query never returns user_id. I tried to echo result of this query and I don't get anything. Database is ok, all other queries works fine. If I run this query in mysql workbench again it works. I don't know where is the problem. Can you help me.

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Answer 1

Remove quotes around user_id, users and username.

Quotes are only for string

mysql_query("SELECT user_id FROM users WHERE username = '$username'")

or optimized

mysql_query('SELECT user_id FROM users WHERE username = "' . $username . '";')