SQL Injections
A SQL injection is a form of security vulnerability that will let a hacker get into the database through the use of vulnerability that he finds in your code. There are many PHP programs that employ MySQL databases as a part of the whole program. By knowing what things can bring a level of vulnerability in your program you can make the proper plans on what to avoid and how to make the whole code more secure.
There is a useful SQL injection cheat sheet available online that contains a section where the vulnerabilities of the PHP and MySQL are listed. You can use this cheat sheet to see if you’re breaking anything, which will help in creating a more secure code.