I am in the process of writing my first plugin and my call to the plugin_basename(__FILE__) function results in a:

PHP Fatal error: Call to undefined function plugin_basename()

My plugin resides in a directory of its own under the standard wp-content/plugins/. I am also using wordpress 3.3.1 and have looked at the codex but cannot see anything wrong. The code block where it fails is shown below:

<?php
/*
Plugin Name: Send Invitation
Plugin URI: http://w3boutique.net
Description: Emails a the link of the current page to a friend
Author: Nandakumar Chandrasekhar
Version: 0.1wp-includes/plugin.php
Author URI: http://w3boutique.net/about-nanda.html
License: GPL2
*/

define( 'SEND_INVITATION_VERSION', '0.1' );

define( 'SEND_INVITATION_REQUIRED_WP_VERSION', '3.3.1' );

// Check if following are required

if ( ! defined( 'SEND_INVITATION_PLUGIN_BASENAME' ) ) {
    define( 'SEND_INVITATION_PLUGIN_BASENAME', plugin_basename(__FILE__) );
 }

if ( ! defined( 'SEND_INVITATION_PLUGIN_NAME' ) ) {
    define( 'SEND_INVITATION_PLUGIN_NAME', trim( dirname(     SEND_INVITATION_PLUGIN_BASENAME ), '/' ) );
}

if ( ! defined( 'SEND_INVITATION_PLUGIN_DIR' ) ) {
    define( 'SEND_INVITATION_PLUGIN_DIR', WP_PLUGIN_DIR . '/' . SEND_INVITATION_PLUGIN_NAME );
}

if ( ! defined( 'SEND_INVITATION_PLUGIN_URL' ) ) {
define( 'SEND_INVITATION_PLUGIN_URL', WP_PLUGIN_URL . '/' . SEND_INVITATION_PLUGIN_NAME );
}

// Version check
global $wp_version;

// Message to send if version is below the required version
$exit_msg = 'Send Invitation requires WordPress or newer' .   SEND_INVITATION_REQUIRED_WP_VERSION . ' <a href="http://codex.wordpress.org/Upgrading_WordPress">Please update!</a>';

if ( version_compare( $wp_version, SEND_INVITATION_REQUIRED_WP_VERSION, "<" ) ) {
    exit( $exit_msg );
}

function add_send_invitation_link() {
    wp_enqueue_script('jquery');
    wp_enqueue_script( 'show_form.js', SEND_INVITATION_PLUGIN_URL . '/js/show_form.js' );
    wp_enqueue_style( 'send-invitation.css', SEND_INVITATION_PLUGIN_URL . '/css/send-invitation.css' );

    # TODO Put this in another file and include it
    $invitation_form = '
    <h1><a href="#" onclick="show_form();">Send Invitation</a></h1>
    <form id="invitation-form" method="post" action="javascript:void(0);">
        <p>
            Your Email Address<br />
            <input type="text" name="email" />
        </p>
        <p>
            ' . "Friend's Email Address" . '<br />
            <input type="text" name="email" />
        </p>
        <p>
            Subject<br />
            <input type="text" name="subject" />
        </p>
        <p>
            Message<br/>
            <textarea rows="10" cols="20"></textarea>
        </p>
        <p><input type="submit" name="submit" value="Send" /></p>
     </form>
     <script type="text/javascript">
          jQuery("#invitation-form").submit(function(){
          alert("got here");
          var str = "this";
          jQuery.ajax("' . SEND_INVITATION_PLUGIN_URL . '/form-processor.php?action=send_email", str, function(result){
        alert(result);
        });
        alert("finished");
        return(false);
    });
   </script>';

   return $invitation_form;
}

function send_email() {
     error_log("Inside send email", 0);
     $subject = 'test';
     $message = 'this is a test from send invitation';
     $to = '[email protected]';
     wp_mail($to, $subject, $message);
}

?>

I would appreciate any pointers you may have to what I might be doing wrong here.

Thanks in advance.