How to prevent SQL injection in PHP?

[Jack Hard]

If user input is inserted into an SQL query directly, 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;--, making the query:

INSERT INTO table (column) VALUES('value'); DROP TABLE table;--')

What should one do to prevent this?