Use WordPress' is_page() To Display Custom Content
Something that comes up on a regular basis is how to display certain content on different pages or if certain conditions are met. One way to accomplish this would be to create separate templates for each page. However, if you have a bunch of pages this can quickly become a pain. The other way to accomplish this would be to use WordPress' is_page() function to display content when you want.
First let's take a look at what the function is. According to the codex
This Conditional Tag checks if Pages are being displayed. This is a boolean function, meaning it returns either TRUE or FALSE.
Ok, that's not the most helpful description ever. A simpler explanation would be that the tag checks the name of the page and returns TRUE or FALSE. This means you can put a nice little echo in there and have it display whatever you want.
So let's say you have one page.php file. This is the template you use on every page on your site. On your about page you would like to add a secondary navigation. I don't know why you want to, let's just say you do. This navigation sits above the content area, so you can't just add it into your page through the WordPress wysiwyg editor. Again, you could create a new template for the page, but that is boring. Instead, where you want the second navigation to be, enter this into your page.php file.
The code is asking if the page is named "about", if it is, it echo's the div "secondnav". If the page isn't called "about" then it echo's nothing and nothing is displayed. It's that simple. If you want the nav to display on the "about"and "contact"page add this
This works well for pages, but for your blog page and single posts you will need a different function.
For your blog page use
For your single posts use
WordPress provides a slew of other tags as well including
is_home(), is_front_page(), is_search(), is_404(), is_singular(), is_page(), is_attachment(), is_local_attachment(), is_single(), is_sticky(), is_archive(), is_category(), is_tag(), is_author(), is_date(), is_year(), is_month(), is_day(), is_time(), is_admin(), is_preview(), is_paged(), is_page_template(), is plugin active(), is_plugin_page(), is_new_day(), is_feed(), is_trackback(), is_comments_popup(), comments_open(), pings_open(), is_taxonomy(), is_taxonomy_hierarchical(), is_term(), is_user_logged_in(), is_blog_installed(), is_active_sidebar(), is_dynamic_sidebar(), is_active_widget()
As you can see, you can target pretty specific events and conditions. So have fun experimenting with conditional tags and if you come up with something cool, let me know in the comments.
Edit: To use regular PHP and not WordPress to include content, see my post here.