Let's say we have a query like this:

$query = "SELECT * FROM test_table WHERE id=$id";

This is vulnerable to SQL Injection because the variable $id has not protection.
To avoid this we can use a PHP function, intval.
To see what it does go here:
http://www.php.net/intval

So if we use it:

$safe = intval($id);
$query = "SELECT * FROM test_table WHERE id=$safe";

Another way is to make our query like this:
$query = "SELECT * FROM test_table WHERE id='".$id."'";

Another way also is to use is_numeric():

if (is_numeric ($_GET['id']) ) {
$query = "SELECT * FROM test_table WHERE id={$_GET['id']}";
} else { echo"Again nooby"; exit; }