SQL injection is a security vulnerability that allows attackers to execute arbitrary SQL commands on a database server by manipulating input data. It can occur when user-provided input is not properly sanitized before being used in an SQL query, allowing the attacker to inject malicious SQL code into the query.

To prevent SQL injection in PHP, you should follow these best practices:

1. Use prepared statements and parameterized queries: Prepared statements and parameterized queries allow you to separate the SQL code from the data being provided by the user, which prevents the user’s input from being interpreted as part of the SQL code. Prepared statements are supported by most database libraries in PHP, such as PDO and MySQLi.
2. Escape special characters: Special characters such as single quotes and backslashes can be used to terminate a string or to inject additional SQL code into a query. You should escape these characters by adding a backslash before them when they are used in user-provided input.
3. Validate user input: You should validate user input to ensure that it is in the expected format and contains no malicious code. For example, you can use regular expressions to check that a string contains only alphabetical characters, or you can check that a number falls within a certain range.
4. Use whitelists: Instead of trying to identify and block all malicious input, you can create a whitelist of allowed characters or values and only accept input that matches the whitelist. This can be more effective than using blacklists, as it is easier to identify allowed input than to identify all possible malicious input.
5. Use stored procedures: Stored procedures are pre-defined SQL code that is stored on the server and can be executed by calling a stored procedure name. Stored procedures can help to prevent SQL injection by separating the SQL code from the data provided by the user, and by allowing you to define strict input and output parameters.
6. Use an ORM (Object-Relational Mapper): An ORM is a library that abstracts away the details of the database and allows you to write code in a high-level language, such as PHP, rather than SQL. This can help to prevent SQL injection by eliminating the need to write raw SQL queries and by providing automatic parameterization and escaping of user-provided input.

By following these best practices, you can significantly reduce the risk of SQL injection in your PHP code and protect your database from malicious attacks.