Compatibility Code

Compatibility code is code that serves to "hide" differences in coding to accommodate different versions, such as different PHP versions or different MySQL versions. By providing compatibility code with a public interface we can maintain a clean and simple API while dealing with version differences automatically.

Different types of compatibility code

Compatibility code can be classified into three kinds, depending on what kind of differences we are hiding; the aim is always to hide the complexity of working around differences from calling code that needs the functionality:


Missing functions
Hiding decision logic

Compatibility code in Wikka

Wikka already has some compatibility code in place, but it could do better... See below for some (proposed) new additions.


Missing function.

The function mysql_real_escape_string() is available in PHP as of version 4.3; Wikka specifies PHP 4.1.0 as minimum PHP version so we need to provide a (near) equivalent for this.

The code is found near the start of wikka.php:
  1. if ( ! function_exists("mysql_real_escape_string") )
  2. {
  3.     function mysql_real_escape_string($string)
  4.     {
  5.         return mysql_escape_string($string);
  6.     }
  7. }

This clearly shows the basic pattern for "missing function" compatibility code: This ensures that the intended function can be called as normal: if the function does not exist in the current installation, the compatibility code will kick in.

Note that mysql_escape_string() is not exactly equivalent with mysql_real_escape_string(); possibly the code here could be extended a bit to provide better compaitibilty.

Magic quotes


PHP may be configured to use "magic quotes". When magic_quotes are on, all ' (single-quote), " (double quote), \ (backslash) and NUL's are escaped with a backslash automatically for GPC (Get/Post/Cookie) operations. To avoid having to check for these backslashes in every string we get via cookies or POST and GET requests the following code turns off magic quotes and removes them where found, so any further code does not have to deal with them. The code is (currently) found near the end of wikka.php:
  1. // workaround for the amazingly annoying magic quotes.
  2. function magicQuotesSuck(&$a)
  3. {
  4.     if (is_array($a))
  5.     {
  6.         foreach ($a as $k => $v)
  7.         {
  8.             if (is_array($v))
  9.                 magicQuotesSuck($a[$k]);
  10.             else
  11.                 $a[$k] = stripslashes($v);
  12.         }
  13.     }
  14. }
  15. {
  16.     magicQuotesSuck($_POST);
  17.     magicQuotesSuck($_GET);
  18.     magicQuotesSuck($_COOKIE);
  19. }

In a future version this would be better placed together with other compatibility code near the start and maybe get a more "polite" name. :)

Proposed new compatibility code

This is compatibility code needed for or used used by new code development presented on this site.


Installed to support a beta feature on this server as of 2005-06-12.

Missing function.

The function html_entity_decode() is available in PHP as of version 4.3. The "advanced" formatter makes use of it when generating ids for headings. The following code provides a near equivalent. Insert in wikka.php immediately after the mysql_real_escape_string() compatibility code shown above:

  1. if (!function_exists('html_entity_decode'))
  2. {
  3.     // based on
  4.     // third parameter (charset) ignored: only (default) ISO-8859-1 character set supported
  5.     function html_entity_decode($string,$quote_style=ENT_COMPAT)
  6.     {
  7.         $trans_tbl = get_html_translation_table(HTML_ENTITIES,$quote_style);
  8.         $trans_tbl = array_flip($trans_tbl);
  9.         return strtr($string, $trans_tbl);
  10.     }
  11. }

Note that it is not completely equivalent: html_entity_decode() in PHP 4.3+ provides support for a range of character sets (including UTF-8); the character set can be specified in a third parameter. The function get_html_translation_table() we use to provide compatibility only uses the default charset ISO-8859-1; when this code is used in versiosn of PHP lower than 4.3 any third parameter will be ignored.

Note that for the case this was written (generating ids for headings) we actually need ISO-8859-1, so for that application lack of equivalence it is not a problem; used in another context it may be and one should be aware of the limitations.


Installed to support a beta feature on this server as of 2005-06-12 (including the efficiency change).

Hiding decision logic.

The current (version category action uses the following bit of code to gather the the members of a category:

  1.     if ($this->CheckMySQLVersion(4,0,1))
  2.     {
  3.             $results = $this->FullCategoryTextSearch($page);
  4.     }
  5.     else
  6.     {
  7.             $results = $this->FullTextSearch($page);
  8.     }

This type of decision logic doesn't belong in an action (or a handler for that matter): if we'd decide to support a different set of versions of PHP and MySQL we'd have to trail through all of our code to find ths kind of decision logic. Instead, there should be a single method call to a method that hides the decision process about what function to use when.

So for the advanced category action this code was spirited away in a new core function getCatMembers() - insert this right after FullCategoryTextSearch() in wikka.php at line 438:

  1.     /**
  2.      * Find category members, hiding version-dependent implementation from caller.
  3.      *
  4.      * @uses    CheckMySQLVersion()
  5.      * @uses    FullCategoryTextSearch()
  6.      * @uses    FullTextSearch()
  7.      *
  8.      * @todo    - replace FullTextSearch() with a query that returns only tag
  9.      *          - replace CheckMySQLVersion() with PHP's generiic function
  10.      *
  11.      * @param   string  $category   required: category to find members of.
  12.      * @return  array   rows with page data (for pages that refer to the category name)
  13.      */
  14.     function getCatMembers($category)
  15.     {
  16.         if ($this->CheckMySQLVersion(4,0,1))
  17.         {
  18.             return $this->FullCategoryTextSearch($category);
  19.         }
  20.         else
  21.         {
  22.             return $this->FullTextSearch($category);        # @@@ could be more efficient by returning only 'tag'
  23.         }
  24.     }

This not only hides which function to call, but also how that decision is made. In fact, it's now completely hidden how category members are gathered at all: if we'd install a different system such as a categories table, the content of the method would change to query that table, but the category action would not need to be changed at all.

The method FullCategoryTextSearch() is (indirectly now via getCatMembers()) used only by the category action. All the action needs to work with is page names - but if we look at the query used by the action we see it's actually retrieving all columns (*), including the whole page body (line 438 in the release version):
    function FullCategoryTextSearch($phrase) { return $this->LoadAll("select * from ".$this->config["table_prefix"]."pages where latest = 'Y' and match(body) against('".mysql_real_escape_string($phrase)."' IN BOOLEAN MODE)"); }

It's only a trivial change to get only the page name ('tag'):
    function FullCategoryTextSearch($phrase) { return $this->LoadAll("SELECT tag FROM ".$this->config["table_prefix"]."pages WHERE latest = 'Y' AND MATCH(body) AGAINST('".mysql_real_escape_string($phrase)."' IN BOOLEAN MODE)"); }

Looking at FullTextSearch(), this is not only used (indirectly) by the category action but also the textsearch and textsearchexpanded actions. This method also asks for all columns (*) but only the textsearchexpanded action actually uses the 'body' field as well as the 'tag' field. This could be made more efficient (for the textsearch action at least) by adding an extra parameter to the method to tell it which field(s) to retrieve. However, that would also involve a change in at least one of those actions; that is outside the scope of this page.

Refer to WikkaOptimization for more optimization issues.

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