PHP Code:
$Words = $frow["descrizione"];
//lets assume your keywords are in an array called $KeyWords
foreach ($KeyWords as $KeyWord)
{ $Words = str_replace($KeyWord,"<span class='bold'>".$KeyWord."</span>",$Words);
}
so descrizione contains the text you want to highlight anmd is copied to $Words
you have an array $KeyWords with each of the words you want to highlight
the foreach statement iterates through every element in the array
the $Words text is scanned for each occurance of a key word, where found it adds the 'span' tags, and then moves onto the next keyword untill the array is exhausted.
having done that you have a variable $words which can be treated/handled as you wish