When dealing with PHP and MySQL, the “Uncaught error: Call to undefined function mysql_connect()” problem may be quite frustrating. This error typically occurs when your code is trying to use the mysql_connect() function, which is no longer available in modern PHP versions.

Fatal error: Uncaught Error: Call to undefined function mysql_connect() 

It occurs when you attempt to establish a connection to a MySQL database server using the deprecated mysql_connect() function in PHP. The error message is as followed:

The article will explain what this issue is, why it occurs, and, most importantly, how to solve it. We’ll also look at effective practises for avoiding this issue in the future. 

What is the Error?

The error message “Uncaught error: Call to undefined function mysql_connect()” indicates that your PHP script is attempting to use the mysql_connect() function, but PHP does not recognise it as a valid function. This issue frequently happens when you attempt to connect to a MySQL database using the deprecated mysql_connect() function, which was removed in PHP 7.0. 

Causes of the Error

This error can occur in various situations, but it primarily happens when you are trying to establish a connection to a MySQL database using the deprecated mysql_connect() function. Here are some reasons:

1. Using mysql_connect() with Modern PHP Versions

In this example, we have a PHP script that attempts to connect to a MySQL database using the mysql_connect() function. This code was written for older PHP versions where mysql_connect() was supported. However, in modern PHP versions (PHP 7 and later), this function is deprecated and removed, leading to the “Uncaught error: call to undefined function mysql_connect()” error.

2. Outdated CMS or Third-Party Code

In this case, the code is from a content management system (CMS) or a third-party application. To create a database connection, the code employs the deprecated mysql_connect() method. If you try to execute this code on a server running a newer PHP version, you will get the “Uncaught error: call to undefined function mysql_connect()” error since the function is undefined in current PHP.

Solutions to Fix the Error

To fix this issue, you need to transition to using either MySQLi or PDO for database connections, as these are the modern and supported methods.

1. Using MySQLi

To resolve this error, you need to replace the deprecated mysql_connect() function with the modern MySQLi (MySQL Improved) extension. Here’s the code:

connect_error) {
    die('Connection failed: ' . $mysqli->connect_error);
}
// Perform a sample query
$query = "SELECT * FROM table_name";
$result = $mysqli->query($query);
// Check for query success
if (!$result) {
    die('Query failed: ' . $mysqli->error);
}
// Fetch and process the result set
while ($row = $result->fetch_assoc()) {
    // Your code for processing each row will come here
}
// Closing the database connection when done
$mysqli->close();
?>

In the above example, to establish a database connection, we create a new MySQLi object ($mysqli) . We afterwards run an example query and properly handle both connection and query problems. Finally, we receive and analyze the result set before closing the database connection.

2. Using PDO

Another way to solve this issue is to use the PDO extension.

setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    die('Connection failed: ' . $e->getMessage());
}
// Performing a sample query
$query = "SELECT * FROM table_name";
try {
    $stmt = $pdo->query($query);
    // Fetching and analyzing the result set
    while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
        // Your code for processing each row will come here
    }
} catch (PDOException $e) {
    die('Query failed: ' . $e->getMessage());
}
?>

 In the PDO example, we use a try-catch block to handle exceptions that may occur during the database connection and query. To establish a database connection, we construct a PDO object ($pdo) and configure it to raise exceptions on faults. Then we run a test query, get and process the result set, and handle any errors that may arise during these processes.

3. Connecting the MySQLi with a PDO object

Connecting to a MySQL database using both MySQLi and PDO in the same PHP script is possible. The code is as shown:

connect_error) {
    die('MySQLi Connection failed: ' . $mysqli->connect_error);
}
// Creating PDO Connection
try {
    $pdo = new PDO('mysql:host=localhost;dbname=database_name', 'username', 'password');
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    die('PDO Connection failed: ' . $e->getMessage());
}
// Using MySQLi
$mysqliQuery = "SELECT * FROM table_name";
$mysqliResult = $mysqli->query($mysqliQuery);
if (!$mysqliResult) {
    die('MySQLi Query failed: ' . $mysqli->error);
}
while ($row = $mysqliResult->fetch_assoc()) {
    // Process data fetched using MySQLi
    echo 'MySQLi Data: ' . $row['column_name'] . '
';
}
// Using PDO
$pdoQuery = "SELECT * FROM table_name";
try {
    $pdoStatement = $pdo->query($pdoQuery);
    while ($row = $pdoStatement->fetch(PDO::FETCH_ASSOC)) {
        // Process data fetched using PDO
        echo 'PDO Data: ' . $row['column_name'] . '
';
    }
} catch (PDOException $e) {
    die('PDO Query failed: ' . $e->getMessage());
}
// Close MySQLi connection
$mysqli->close();
// PDO connection is automatically closed when the script ends
?>

In this example, first we have established connections using both MySQLi and PDO then we performed queries and fetch data separately for each connection method. After fetching data, we used error handling for both connections and queries.

4. Integrating the custom code and WordPress plugins

The solution involves determining whether a website is built using WordPress or custom code.  Compatibility with the latest PHP versions can be an issue for WordPress sites, particularly with older themes and plugins. Enabling debugging in the 'wp-config.php' file helps you to determine whether your existing plugins and themes are compatible with PHP 7+ versions. If any problems develop, the answer is generally straightforward: update your plugins and themes to the most recent versions to guarantee they comply with contemporary PHP standards. For ultimate performance and safety, it may be important to replace outdated plugins or themes with more recent alternatives in some circumstances.

For websites built with custom code, a manual inspection of the codebase becomes essential. Syntax problems or misspelt functions can frequently result in the “Uncaught error” error. While the lack of adequate MySQLi functions is a typical cause, referring to the previous solution offered insight on how to resolve this issue.

By following these procedures and making the required adjustments, you can ensure that your website operates smoothly, is error-free, and is compatible with current PHP versions.

5. Rolling back to older PHP versions and updating the code

Rolling back to older PHP versions and updating the code is a potential approach to address the “Uncaught error: call to undefined function mysql_connect()” error, especially when dealing with legacy code that relies on deprecated PHP functions. However, this approach comes with both advantages and disadvantages:

Advantages:

1. Rolling back to a previous PHP version that supports deprecated functions such as mysql_connect() can immediately resolve the problem because the functions are now available again.
2. This method might involve fewer code changes because you may continue to use the deprecated functions without having to immediately update your codebase.
3. Modern database connection methods provide enhanced security and protection against common vulnerabilities such as SQL injection.

Disadvantages:

1. Since older PHP versions may not receive security updates or fixes, they may expose your website to security vulnerabilities and dangers. This can endanger your website and data.
2. Newer libraries, plugins, and themes may need capabilities and functions that are only accessible in the most recent PHP releases as PHP continues to develop. Rolling back may lead to compatibility issues with other components of your website.
3. Rolling back PHP is only a short-term solution. You’ll eventually need to upgrade your code to work with current PHP versions because older versions will stop receiving support.

Wrapping Up

The “Uncaught error: call to undefined function mysql_connect()” error might be concerning but it is an issue that can be solved. Understanding why this happens and how to fix it is critical for keeping your PHP apps functioning and secure. You may assure a smoother development experience and a more secure online application by switching to contemporary database connection technologies like MySQLi or PDO and following best practises:

1. Regularly update your PHP version to ensure that you get the most latest security fixes and compatibility enhancements.
2. Check the official documentation or support resources of any third-party plugins to confirm that they use the most recent database access techniques.
3. Use version control systems like Git to track changes to your custom code. This allows you to conveniently maintain code versions, communicate with others, and reverting to prior versions if necessary.