How to diagnose and resolve PHP database connection issues?

Diagnosing and resolving PHP database connection issues Confirm that the connection information (hostname, username, password) is correct. Use mysqli_connect_errno() and mysqli_connect_error() to get error information. Check the log file for details. Make sure that the firewall allows connections to the port of the database server (usually 3306). Check if the database server is running.

How to diagnose and solve PHP database connection problems

Database connection problems are common errors in PHP development. This article will guide you on how to diagnose and resolve these issues.

Step 1: Check the connection information

The basic information required to connect to the database includes:

• Host name or IP address
• Database Name
• Username
• Password

Make sure all this information is correct and you have the necessary access rights.

Step 2: Use mysqli_connect_errno() and mysqli_connect_error()

##mysqli_connect() The function will return a Boolean value indicating whether the connection is successful. If the connection fails, you can get the error code and message using the mysqli_connect_errno() and mysqli_connect_error() functions.

Example:

$link = mysqli_connect("hostname", "username", "password", "database");

if (!$link) {
    $error_code = mysqli_connect_errno();
    $error_message = mysqli_connect_error();
}

Step 3: Check the log file

Logging may be enabled in your PHP configuration. Check the log file for details about the connection problem. The log file is usually located in

/var/log/php-fpm.log or /var/log/php.log.

Step 4: Firewall and Port

Make sure the firewall allows the port connecting to the database server. Typically, MySQL server uses port 3306.

Step 5: Database server status

Check whether the database server is running. You can use the following command:

sudo service mysql status

If the server is not running, use the following command to start it:

sudo service mysql start

Practical case

Question ：mysqli_connect(): (HY000/1130): Host 'localhost' is not allowed to connect to this MySQL server error occurs when connecting to the MySQL database.

Solution: Check the firewall rules and find that MySQL port 3306 is not allowed. Allow the port in the firewall and the problem is solved.

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