“The Exec-PHP plugin executes <?php ?> code in your posts, pages and text widgets.” -Source

I downloaded this plugin a couple of days ago so I could post the code converter script, which I got from the wonderful site TutorialTastic. I must admit it was a bit troublesome to install, but worth it!

You can download this plugin here. Download, unzip, and upload the exec-php folder on your plugins directory. Go to Wordpress and activate it!

Now comes the hard part! Check if you have done the following before you test the plugin:

• Disable the WYSIWYG editor through your profile. Go to Users » Your Profile. At the very top you will see the following:
  
  Make sure the box is not checked. Having it on and just switching to the HTML tab will not work. You could try out other Plugins that turn off the visual editor for certain articles¹, such as the Deactivate Visual Editor plugin² and the Disable Visual Editor plugin.
• Next, go to Settings » Writing. When it loads, scroll to the bottom of the page and verify the box next to “WordPress should correct invalidly nested XHTML automatically” in not checked.
• You might³ need to install the Role Manager Plugin so you can allow yourself to write ‘unfiltered_html’.

Choose a post in which you’ll test if the plugin works or not. We’ll use the following text to try to execute a <?php ?> command:

 <?php echo "This is the Exec-PHP 'Hello World'"; ?> 

After you save the post, view it. It should be displayed like this: This is the Exec-PHP 'Hello World' .

Now, enjoy having the capability to use PHP scripts on your Wordpress Powered site!
Rating:
Because of the bit of trouble installation might cause.

1. Articles you’ll add PHP coding to [↩]
2. I haven’t used this plugin but I see no posts in the WP Support Forums about this plugin [↩]
3. Don’t install this just yet, since it might not be needed [↩]