Standard Filters Overview – Using Zend Framework 3

8.2. Standard Filters Overview

Standard filters implementing the FilterInterface interface belong to Zend\Filter namespace ²⁶. A filter class inheritance diagram is shown in figure 8.1. From that figure, you can see that base concrete class for most standard filters is the AbstractFilter class, which implements the FilterInterface interface ²⁷.

Figure 8.1. Filter class inheritance

²⁶⁾ In this section, we only consider the standard filters belonging to the Zend\Filter namespace, although there are other filters that can also be considered standard. For example, the Zend\Filter\File namespace contains several filters applicable to processing file uploads (those filters will be covered in the next chapters). Additionally, the Zend\I18n component defines several filter classes that are aware of the user's locale.

²⁷⁾ From figure 8.1, you may also notice that there are several more base filters: AbstractUnicode filter is the base class for the StringToUpper and StringToLower filters, because it provides the string conversion functionality common to both of them. And, the Decompress filter inherits from the Compress filter, because these filters are in fact very similar. By analogy, the Decrypt filter inherits from the Encrypt filter, because they are the "mirror reflection" of each other as well.

You may notice that there is a strange filter called StaticFilter which does not inherit from AbstractFilter base class. This is because the StaticFilter class is actually a "wrapper" (it is designed to be a proxy to another filter without explicit instantiation of that filter).

Standard filters provided by the Zend\Filter component, along with a brief description of each, are listed in table 8.1.

As you can see from the table, the standard filters can be roughly divided into the following groups:

filters casting input data to a specified type (integer, boolean, date-time, etc.);
filters performing manipulations on a file path (getting the base name, parent directory name, etc.);
filters performing compression and encryption of input data;
filters manipulating string data (case conversion, trimming, character replacement and removal, URL normalizing, etc.); and
proxy filters wrapping other filters (Callback, FilterChain and StaticFilter).

Table 8.1. Standard filters

Class name	Description
`Boolean`	Returns a boolean representation of `$value`.
`ToInt`	Casts the input `$value` to `int`.
`Digits`	Returns the string `$value`, removing all but digit characters.
`ToNull`	Returns `null` if the input value can be treated as null; otherwise returns the `$value` itself.
`DateTimeFormatter`	Takes a date & time string in an arbitrary format and produces a date & time string in a given format.
`BaseName`	Given a string containing the path to a file or directory, this filter will return the trailing name component.
`Dir`	Given a string containing the path of a file or directory, this filter will return the parent directory's path.
`RealPath`	Returns canonicalized absolute pathname.
`Compress`	Compresses the input data with the specified algorithm (GZ by default).
`Decompress`	Decompresses the input data with the specified algorithm (the effect is inverse to the `Compress` filter).
`Encrypt`	Encrypts the input data with the specified cryptographic algorithm.
`Decrypt`	Decrypts the input data previously encrypted with the specified cryptographic algorithm.
`Inflector`	Performs the modification of a word to express different grammatical categories such as tense, mood, voice, aspect, person, number, gender, and case.
`PregReplace`	Performs a regular expression search and replace.
`StringToLower`	Converts the string to lowercase letters.
`StringToUpper`	Converts the string to uppercase letters.
`StringTrim`	Removes white spaces (space, tabs, etc.) from the beginning and the end of the string.
`StripNewlines`	Removes new line characters from string (ASCII codes #13, #10).
`HtmlEntities`	Returns the string, converting characters to their corresponding HTML entity equivalents where they exist.
`StripTags`	Removes tags (e.g., `<a></a>`) and comments (e.g., `<!-- -->`).
`UriNormalize`	Converts a URL string to the "normalized" form and prepends the schema part (e.g., converts www.example.com to http://www.example.com).
`Callback`	Allows to use a callback function as a filter.
`FilterChain`	Allows to organize several filters in a chain.
`StaticFilter`	Returns a value filtered through a specified filter class without requiring separate instantiation of the filter object.