You want to filter all input prior to use.
Initialize an empty array in which to store filtered data. After you’ve proven that something is valid, store it in this array:
$filters = array('name' => array('filter' => FILTER_VALIDATE_REGEXP,
'options' => array('regexp' => '/^[a-z]+$/i')),
'age' => array('filter' => FILTER_VALIDATE_INT,
'options' => array('min_range' => 13)));
$clean = filter_input_array(INPUT_POST, $filters);
By using a strict naming convention, you can more easily keep up with what input has been filtered.
Always initializing $clean to an empty array ensures that data cannot be injected into the array; you must explicitly add it.
In the preceding code, the call to filter_input_array() initializes $clean to contain only the filtered information.
Once you adopt a technique such as the use of $clean, it is important that you only use data from this array in your business logic.