Preventing SQL injection with PDO is primarily achieved by using prepared statements and parameter binding. This approach separates user input from SQL, making it impossible for malicious SQL to be executed.

The -l (or --syntax-check) option checks PHP scripts for syntax errors without executing them, aiding in early error detection.

The primary advantage of using PDO over traditional MySQL functions in PHP is the improved speed and efficiency. PDO is designed to be more efficient and faster in handling database connections and queries. It also offers features like prepared statements that enhance security.

The 'explode()' function in PHP is used to split a string into an array of substrings using a specified delimiter. This is commonly used for parsing data.

PHP uses a 'session.save_handler' mechanism to control where session data is stored, protecting it from unauthorized access.

The getimagesize() function is used to determine the type of an uploaded file and ensure it's an image type, based on the returned image type information.

When connecting to a MySQL database using PDO, the DSN typically starts with "mysql:" to specify the database type for the connection.

To convert a string to lowercase in PHP, you use the strtolower() function. It's a built-in function that does this conversion.

To allow users to upload multiple documents at once, you should use the multiple attribute on a single file input field. This enables users to select multiple files in a single input.