SSL does provide increase level of security. But remember the word security is so vast these days that it can fall under a lot of things.
You should use SSL when transmitting the following types of data:
Authentication data (e.g., passwords, session cookies)
Data that data stewards have classified as non-public
Data that needs to be protected from modification on its way to or from the server (e.g., user input or a result set from the server)
It is important to remember that although SSL can protect the flow of information between your server and a web browser, using SSL does not offer any additional system level security.
After setting up the certificate, you will see the HTTPS, check the certificate is correct and valid and you should be good to go.
It will not protect you from various other things including bad programming and lack of data verifications. You should see best practices for PHP security such as http://phpsec.org/projects/guide/ or http://php.robm.me.uk/
You can also improve security by carefully configuring the IIS or Apache with various modules. Ensure latest updates applied and follow best practices.