In the physical world, a filter is typically used for removing unwanted portions of input, and the desired portion of the input passes through as filter output (e.g., coffee). In such scenarios, a filter is an operator that produces a subset of the input. This type of filtering is useful for web applications - removing illegal input, trimming unnecessary white space, etc.

This basic definition of a filter may be extended to include generalized transformations upon input. A common transformation applied in web applications is the escaping of HTML entities. For example, if a form field is automatically populated with untrusted input (e.g., from a web browser), this value should either be free of HTML entities or contain only escaped HTML entities, in order to prevent undesired behavior and security vulnerabilities. To meet this requirement, HTML entities that appear in the input must either be removed or escaped. Of course, which approach is more appropriate depends on the situation. A filter that removes the HTML entities operates within the scope of the first definition of filter - an operator that produces a subset of the input. A filter that escapes the HTML entities, however, transforms the input (e.g., "&" is transformed to "&"). Supporting such use cases for web developers is important, and "to filter," in the context of using Zend_Filter, means to perform some transformations upon input data.

Having this filter definition established provides the foundation for Zend_Filter_Interface, which requires a single method named filter() to be implemented by a filter class.

Following is a basic example of using a filter upon two input data, the ampersand (&) and double quote (") characters:

$htmlEntities = new Zend_Filter_HtmlEntities();

echo $htmlEntities->filter('&'); // &
echo $htmlEntities->filter('"'); // "

If it is inconvenient to load a given filter class and create an instance of the filter, you can use the static method Zend_Filter::filterStatic() as an alternative invocation style. The first argument of this method is a data input value, that you would pass to the filter() method. The second argument is a string, which corresponds to the basename of the filter class, relative to the Zend_Filter namespace. The staticFilter() method automatically loads the class, creates an instance, and applies the filter() method to the data input.

echo Zend_Filter::filterStatic('&', 'HtmlEntities');

You can also pass an array of constructor arguments, if they are needed for the filter class.

echo Zend_Filter::filterStatic('"',
                               'HtmlEntities',
                               array('quotestyle' => ENT_QUOTES));

The static usage can be convenient for invoking a filter ad hoc, but if you have the need to run a filter for multiple inputs, it's more efficient to follow the first example above, creating an instance of the filter object and calling its filter() method.

Also, the Zend_Filter_Input class allows you to instantiate and run multiple filter and validator classes on demand to process sets of input data. See Zend_Filter_Input.

echo Zend_Filter::filterStatic(
    '"',
    'MyFilter',
    array($parameters),
    array('FirstNamespace', 'SecondNamespace')
);

Zend_Filter::setDefaultNamespaces(array('FirstNamespace', 'SecondNamespace'));
echo Zend_Filter::filterStatic('"', 'MyFilter', array($parameters));
echo Zend_Filter::filterStatic('"', 'OtherFilter', array($parameters));

When using two filters after each other you have to keep in mind that it is often not possible to get the original output by using the opposite filter. Take the following example:

$original = "my_original_content";

// Attach a filter
$filter   = new Zend_Filter_Word_UnderscoreToCamelCase();
$filtered = $filter->filter($original);

// Use it's opposite
$filter2  = new Zend_Filter_Word_CamelCaseToUnderscore();
$filtered = $filter2->filter($filtered)

The above code example could lead to the impression that you will get the original output after the second filter has been applied. But thinking logically this is not the case. After applying the first filter my_original_content will be changed to MyOriginalContent. But after applying the second filter the result is My_Original_Content.

As you can see it is not always possible to get the original output by using a filter which seems to be the opposite. It depends on the filter and also on the given input.