How can I prevent SQL injection in PHP

Question

The programme  becomes vulnerable to SQL injection if user input is added to a SQL query without change, as in the example below:

$unsafe_variable = $_POST['user_input']; 

mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");

This is due to the user's ability to input items like value'); DROP TABLE table;—, and the query becomes:

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

What steps may be taken to avoid this scenario?