I can't replicate your findings. However, " +'Nokia 5800' " means " find rows that contain the word 'Nokia', but rank rows higher if they also contain '5800' "
To find an exact phrase, you'd need to wrap the phrase in qutoation marks - which might look like this...
$sql = "SELECT product_name,MATCH (product_name) AGAINST ('+\\"".$item_name."\\"' IN BOOLEAN MODE) score FROM product_table ";
Note that I've rewritten the query so you can see the actual score returned by each result!