Prevent SQL injection in PHP?

$str = "<script>alert('Hacked')</script>";
echo htmlentities($str);

How can I prevent SQL injection in PHP?

//Connect
$unsafe_variable = $_POST["user-input"];
$safe_variable = mysql_real_escape_string($unsafe_variable);

mysql_query("INSERT INTO table (column) VALUES ('" . $safe_variable . "')");
//Disconnect

Source: stackoverflow.com

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