If user input is inserted without modification into an SQL query, then the application becomes vulnerable to SQL injection, like in the following example:
$unsafe_variable = $_POST['user_input'];
mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");
That's because the user can input something like
value'); DROP TABLE table;--, and the query becomes:
INSERT INTO `table` (`column`) VALUES('value'); DROP TABLE table;--')
What can be done to prevent this from happening?