Why does the error "Call to undefined function mysql_colse()" occur when uploading files to a web server?

The error "Call to undefined function mysql_close()" occurs when trying to close a MySQL connection using the outdated mysql extension, which has been deprecated as of PHP 5.5. To solve this issue, you should use the mysqli or PDO extension to establish database connections and close them properly. 

// Establishing a MySQLi connection
$mysqli = new mysqli($host, $username, $password, $database);

// Check connection
if ($mysqli->connect_error) {
    die("Connection failed: " . $mysqli->connect_error);
}

// Perform database operations

// Closing the MySQLi connection
$mysqli->close();

Keywords

PHP mysql_close() web server file upload error

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