Php Code Stuck

Hi Guys, im fairly new to php so a hand would be appreciated.

i want a plugin i have to call the custom field that i have created called “thumbnail”,

the current code reads :

if ($image == ”) $image = get_option(‘siteurl’) . ‘/wp-content/uploads/products_img/’;

I tried the code below with no luck

if ($image == ”) $image = get_option(‘siteurl’) . ‘/wp-content/uploads/products_img/<?php echo get_post_meta($post->ID, ‘thumbnail’, true);?>’;

Any ideas will be very much appreciated as ive been banging my head against the wall!