php newbie

i add sime custom fields to an wordpress website and one of them is adsense (users will put their pub-xxxxxxxxxx) -> i want for the users who set this field from profile to show an adsense ad with their id but for thoose who dont to show my "pub-id"
as much as i know i must use if function .... but i dont know how to use it ...

i use this code to show the profile field:

<table class="form-table">

		<tr>
			<th><label for="adsense">Adsense</label></th>

			<td>
				<input type="text" name="adsense" id="adsense" value="<?php echo esc_attr( get_the_author_meta( 'adsense', $user->ID ) ); ?>" class="regular-text" /><br />
				<span class="description">Please enter your adsense "pub-id".</span>
			</td>
		</tr>

	</table>

PHP:

how can i do this? :|

(by the way ... i`m trying to make an open blog - register and start posting ... many cool options for the users ... read postin guide first --- link in my signature)

 

if(empty($id)){$id=$your_id;}

now just insert your variables.

 

can`t make it work ....
this is the extra profile field :

	<table class="form-table">

		<tr>
			<th><label for="twitter">Twitter</label></th>

			<td>
				<input type="text" name="twitter" id="twitter" value="<?php echo esc_attr( get_the_author_meta( 'twitter', $user->ID ) ); ?>" class="regular-text" /><br />
				<span class="description">Please enter your Twitter username.</span>
			</td>
		</tr>

	</table>

PHP:

and this is what i use to display

    <a href="http://twitter.com/<?php the_author_meta('twitter'); ?>"><img border="0" src="/wp-content/themes/mytheme/custom/img/twitter.png" alt="HTML tutorial" width="24" height="24" /></a>

PHP:

where should i put that code?

 

is at least possible?