条件分岐タグ

Introduction

The Conditional Tags can be used in your Template files to change what content is displayed and how that content is displayed on a particular page depending on what conditions that page matches. For example, you might want to display a snippet of text above the series of posts, but only on the main page of your blog. With the is_home() Conditional Tag, that task is made easy.

Note the close relation these tags have to WordPress' Template Hierarchy.

The Conditions For...

All of the Conditional Tags test to see whether a certain condition is met, and then returns either TRUE or FALSE. The conditions under which various tags output TRUE is listed below. Those tags which can accept parameters are so noted.

The Main Page

is_home(): When the main page is being displayed.

A Single Post Page

is_single(): When any single Post page is being displayed.
is_single('17'): When Post 17 is being displayed as a single Post.
is_single('Beef Stew'): When the Post with Title "Beef Stew" is being displayed as a single Post.
is_single('beef-stew'): When the Post with Post Slug "beef-stew" is being displayed as a single Post.

A PAGE Page

This section refer to WordPress Pages, not any generic webpage from your blog.

is_page(): When any Page is being displayed.
is_page('42'): When Page 42 is being displayed.
is_page('About Me'): When the Page with Title "About Me" is being displayed.
is_page('about-me'): When the Page with Page Slug "about-me" is being displayed.

Unfortunately, there is no is_subpage() function yet. There is, however, a workaround.

if(get_the_title($post->post_parent) != the_title(' ' , ' ',false)) { echo "This is a subpage"; }: determines if the executing page is a subpage by checking if it isn't it's own parent (non-subpages (pages) are their own parent). In this case, it will print "This is a subpage" to the browser.

A Category Page

is_category(): When any Category archive page is being displayed.
is_category('6'): When the archive page for Category 6 is being displayed.
is_category('Cheeses'): When the archive page for the Category with Name "Cheeses" is being displayed.
is_category('cheeses'): When the archive page for the Category with Category Slug "cheeses" is being displayed.
in_category('5'): Returns true if the current post is in the specified category id. read more

See also is_archive() and Category Templates.

An Author Page

is_author(): When any Author page is being displayed.
is_author('1337'): When the archive page for Author number 1337 is being displayed.
is_author('Elite Hacker'): When the archive page for the Author with Nickname "Elite Hacker" is being displayed.
is_author('elite-hacker'): When the archive page for the Author with Nicename "elite-hacker" is being displayed.

See also is_archive() and Author Templates.

A Date Page

is_date(): When any date-based archive page is being displayed (i.e. a monthly, yearly, daily or time-based archive).
is_year(): When a yearly archive is being displayed.
is_month(): When a monthly archive is being displayed.
is_day(): When a daily archive is being displayed.
is_time(): When an hourly, "minutely", or "secondly" archive is being displayed.

Any Archive Page

is_archive(): When any type of Archive page is being displayed. Category, Author and Date based pages are all types of Archives.

A Search Result Page

is_search(): When a search result page archive is being displayed.

A 404 Not Found Page

is_404(): When a page displays after an "HTTP 404: Not Found" error occurs.

A Paged Page

is_paged(): When the page being displayed is "paged". This refers to an archive or the main page being split up over several pages. This does not refer to a Post or Page whose content has been divided into pages using the  QuickTag.

A Syndication

is_feed(): When the site requested is a Syndication. This tag is not typically used by users; it is used internally by WordPress and is available for Plugin Developers.

A Trackback

is_trackback(): When the site requested is WordPress' hook into its Trackback engine. This tag is not typically used by users; it is used internally by WordPress and is available for Plugin Developers.

Working Examples

Here are working samples to demonstrate how to use these conditional tags.

Single Post

This example shows how to use is_single() to display something specific only when viewing a single post page:

if (is_single())
{
     echo 'This is just one of many fabulous entries in the ' . single_cat_title() . ' category!';
}

Date-Based Differences

If someone browses our site by date, let's distinguish posts in different years by using different colors:

<?php
// this starts The Loop
if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<h2 id="post-<?php the_ID(); ?>">
<a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title(); ?>">
<?php the_title(); ?></a></h2>
<small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>

<?php
// are we showing a date-based archive?
if (is_date())
{
     if (date('Y') != get_the_date('Y'))
     {
          // this post was written in a previous year
          // so let's style the content using the "oldentry" class
          echo '<div class="oldentry">';
     } else {
          echo '<div class="entry">';
     }
} else {
     echo '<div class="entry">';
}
the_content('Read the rest of this entry »'); 
?>
</div>

Variable Sidebar Content

This example will display different content in your sidebar based on what page the reader is currently viewing.

<!-- begin sidebar -->
<div id="sidebar">
<?php
// let's generate info appropriate to the page being displayed
if (is_home()) {
        // we're on the home page, so let's show a list of all top-level categories
        echo "<ul>";
        wp_list_cats('optionall=0&sort_column=name&list=1&children=0');
        echo "</ul>";
} elseif (is_category()) {
        // we're looking at a single category view, so let's show _all_ the categories
         echo "<ul>";
        wp_list_cats('optionall=1&sort_column=name&list=1&children=1&hierarchical=1');
        echo "</ul>";
} elseif (is_single()) {
        // we're looking at a single page, so let's not show anything in the sidebar
} elseif (is_page()) {
        // we're looking at a static page.  Which one?
        if (is_page('About')) {
             // our about page.
             echo "<p>This is my about page!</p>";
        } elseif (is_page('Colophon')) {
             echo "<p>This is my colophon page, running on WordPress " . bloginfo('version') . "</p>";
        } else {
              // catch-all for other pages
              echo "<p>Vote for Pedro!</p>";
        }
} else {
        // catch-all for everything else (archives, searches, 404s, etc)
        echo "<p>Pedro offers you his protection.</p>";
} // That's all, folks!
?>
<form id="searchform" method="get" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<div>
<input type="text" name="s" id="s" size="15" />
<input type="submit" value="<?php _e('Search'); ?>" />
</div>
</form>

</div>
<!-- end sidebar -->

Helpful 404 page

When a visitor gets a 404 error page, it can be intimidating, and unhelpful. Using Wordpress, you can take the edge off a 404 and make it helpful to users, and yourself, too, by emailing whenever the user clicks a link to a non-existant page. If you use this, don't forget o

<p>You 
<?php
#some variables for the script to use
#if you have some reason to change these, do.  but wordpress can handle it
$adminemail = get_bloginfo('admin_email'); #the administrator email address, according to wordpress
$website = get_bloginfo('url'); #gets your blog's url from wordpress
$websitename = get_bloginfo('name'); #sets the blog's name, according to wordpress

  if (!isset($_SERVER['HTTP_REFERER'])) {
    #politely blames the user for all the problems they caused
        echo "tried going to "; #starts assembling an output paragraph
	$casemessage = "All is not lost!";
  } elseif (isset($_SERVER['HTTP_REFERER'])) {
    #this will help the user find what they want, and email me of a bad link
	echo "clicked a link to"; #now the message says You clicked a link to...
        #setup a message to be sent to me
	$failuremess = "A user tried to go to $website"
        .$_SERVER['REQUEST_URI']." and received a 404 (page not found) error. ";
	$failuremess .= "It wasn't their fault, so try fixing it.  
        They came from ".$_SERVER['HTTP_REFERER'];
	mail($adminemail, "Bad Link To ".$_SERVER['REQUEST_URI'],
        $failuremess, "From: $websitename <noreply@$website>"); #email you about problem
	$casemessage = "An administrator has been emailed 
        about this problem, too.";#set a friendly message
  }
  echo " ".$website.$_SERVER['REQUEST_URI']; ?> 
and it doesn't exist. <?php echo $casemessage; ?>  You can click back 
and try again or search for what you're looking for:
  <?php include(TEMPLATEPATH . "/searchform.php"); ?>
</p>

Dynamic Menu Highlighting

The Dynamic Menu Highlighting article discusses how to use the conditional tags to enable highlighting of the current page in a menu.

関数リファレンス、テンプレートタグ目次もご覧ください。