使用mysqli在config.php中选择数据库失败问题求助

Hey Marc, let's work through this database selection issue you're facing—it's a typical hiccup when moving from the old mysql_* functions to mysqli_*, so we can nail this down step by step.

First, let's start with the most likely culprits:

1. You're mixing mysql_* and mysqli_* functions #

You mentioned you sometimes have to add an "i" to make things work, which suggests your codebase might be using a mix of both legacy and modern MySQL functions. These two APIs don't play nice together—you can't use a connection created with mysql_connect() to run mysqli_select_db(), and vice versa. Make sure all database-related code is consistently using mysqli_* (since mysql_* is deprecated anyway).

2. Missing or incorrect database name in config #

Looking at the config snippet you shared, you have hostname, mysql_user, and mysql_pass, but no entry for the database name itself! You need a line like:

$SETTINGS["mysql_db"] = "your_database_name_here";

Without defining which database you want to select, even a valid connection can't pick one. Double-check that this setting exists and matches the exact name of your MySQL database (case-sensitive on some systems like Linux).

3. Wrong parameter order for mysqli_select_db() #

Unlike the old mysql_select_db($dbname, $conn), the mysqli procedural version flips the parameters: it's mysqli_select_db($connection, $dbname). If you're copying the old function's parameter order, that'll cause a failure every time.

Here's how a correct procedural mysqli setup should look:

// Create connection
$conn = mysqli_connect($SETTINGS["hostname"], $SETTINGS["mysql_user"], $SETTINGS["mysql_pass"]);

// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Select database
if (!mysqli_select_db($conn, $SETTINGS["mysql_db"])) {
    die("Unable to select database: " . mysqli_error($conn));
}

If you prefer the object-oriented style (cleaner in my opinion), it would be:

$conn = new mysqli($SETTINGS["hostname"], $SETTINGS["mysql_user"], $SETTINGS["mysql_pass"]);

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

if (!$conn->select_db($SETTINGS["mysql_db"])) {
    die("Unable to select database: " . $conn->error);
}

4. Permission issues (even though you can connect) #

Just because you can log in with root doesn't mean that user has access to your target database. To verify, log into MySQL directly and run:

SHOW GRANTS FOR 'root'@'localhost';

Look for a line like GRANT ALL PRIVILEGES ON your_database_name.* TO 'root'@'localhost';—if it's missing, you'll need to grant access:

GRANT ALL PRIVILEGES ON your_database_name.* TO 'root'@'localhost';
FLUSH PRIVILEGES;

Pro Tip: Always Show Specific Errors #

Instead of just outputting "Unable to select database.", use mysqli's built-in error functions to get the exact MySQL error message. This will tell you if the database doesn't exist, you don't have permissions, or there's a typo—way more helpful than a generic message.

Give these steps a try, and let me know if you hit any specific error messages after enabling detailed debugging!

内容的提问来源于stack exchange，提问作者Marc