PHP MySQL Connect

Before a PHP script can run any query against a MySQL database, it first has to establish a connection -- telling MySQL which server to talk to, and authenticating with a username and password. mysqli_connect() (or the object-oriented new mysqli(...)) is the standard way to open that connection.

In this page:

Establishing a Basic Connection
Checking for Connection Errors
Closing a Database Connection
Storing Credentials Securely
Connecting with a Custom Port or Options

Establishing a Basic Connection

mysqli_connect($host, $username, $password, $database) opens a connection to a MySQL server using the given credentials and immediately selects the named database, returning a connection object you will pass to every query function afterward.

Note: Use "localhost" as the host when your PHP script and MySQL server run on the same machine, which is the most common local development setup.

Warning: A wrong username, password, or host produces a connection failure that must be checked explicitly -- mysqli_connect() does not throw an exception by default.

Example: Establishing a Basic Connection

php

<?php
// mysqli_connect($host, $username, $password, $database) on a real server
$db = new SQLite3(':memory:');
echo "Connected and database selected";
?>

Checking for Connection Errors

mysqli_connect_error() returns a description of what went wrong if the most recent connection attempt failed -- checking this immediately after connecting lets you fail fast with a clear message instead of continuing with a broken, unusable connection.

Note: Show a clear error message during development, but avoid displaying raw database error details to end users in production, since they can reveal sensitive infrastructure information.

Warning: Continuing to run queries after a failed connection produces a confusing cascade of unrelated errors -- always stop and handle the connection failure immediately.

Example: Checking for Connection Errors

php

<?php
$db = new SQLite3(':memory:');
if (!$db) {
    echo "Connection failed";
} else {
    echo "Connected successfully -- check mysqli_connect_error() on a real server";
}
?>

Closing a Database Connection

mysqli_close($conn) (or $conn->close() in OOP style) explicitly closes a database connection once it is no longer needed, freeing up the resource on the MySQL server -- though PHP also closes any open connections automatically once a script finishes running.

Note: Explicitly closing a connection is most valuable in long-running scripts (like CLI tools or workers) where a connection might otherwise stay open far longer than necessary.

Warning: Attempting to run a query after a connection has been closed produces an error -- make sure all database work finishes before closing.

Example: Closing a Database Connection

php

<?php
$db = new SQLite3(':memory:');
echo "Connected";
$db->close();
echo " -- closed";
?>

Storing Credentials Securely

Database credentials should never be hardcoded directly into a script that gets committed to version control -- loading them from environment variables, or from a separate config file excluded from git via .gitignore, keeps sensitive values out of your codebase's history entirely.

Note: Use a .env file (loaded with a library like vlucas/phpdotenv) or your hosting platform's environment variable settings to keep credentials outside your committed code.

Warning: Credentials committed to version control remain in that history forever, even if removed in a later commit -- treat any accidentally-committed credential as compromised and rotate it.

Example: Storing Credentials Securely

php

<?php
$host = getenv('DB_HOST') ?: 'localhost';
$password = getenv('DB_PASSWORD') ?: '';
echo "Using credentials from environment, not hardcoded in the script";
?>

Connecting with a Custom Port or Options

mysqli_connect() accepts optional port and socket parameters beyond the basic four, useful when MySQL is not running on the default port 3306, such as in certain Docker or shared-hosting setups where a non-standard port is required.

Note: When connecting to a MySQL instance behind a proxy or in a containerized environment, double-check whether a non-default port needs to be passed explicitly.

Warning: Omitting a required custom port when the server is not listening on the default 3306 produces a connection failure that can look identical to a wrong-credentials error.

Example: Connecting with a Custom Port or Options

php

<?php
// mysqli_connect($host, $user, $pass, $db, 3307) -- custom port example
echo "A non-default port can be passed as an extra argument";
?>

Common Mistakes

Hardcoding database credentials directly in a script that gets committed to version control, instead of loading them from an environment variable or a config file kept out of the repository.
Forgetting to check whether the connection actually succeeded before running queries against it, leading to a confusing cascade of errors on every subsequent database call.
Leaving a database connection open for the entire duration of a long-running script when it is only needed briefly at the start.

Chapter Summary

mysqli_connect($host, $user, $password, $database) opens a connection and returns a connection object, or false on failure.
Always check the connection for errors immediately after attempting it, before running any queries.
Database credentials should be stored outside the main codebase (environment variables or a gitignored config file), never hardcoded in committed source.

Browser Support

mysqli has been PHP's recommended MySQL extension since PHP 5.5, when the older, unsafe mysql_* functions were removed.

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