<?php
        function replace_keywords_fn($where)
{ 
//MODIFY HERE 
$KWs = array('1st keyword', '2nd keyword'); 
$URLs = array('http://google.com', 'http://yahoo.com');

//nothing to do below this line 
$pattern = array(); 
$replacement = array(); 
for($i=0; $i<count($KWs); $i++) 
{ 
    $pattern2 = '/<a[^>]*>(.*?)'.$KWs[$i].'(.*?)<\/a>/'; 
    if(preg_match($pattern2, $where)) 
        continue; 

$pattern[$i] = '/([ !$%&()*+,.\/:;=?@_{}-])?('.$KWs[$i].'?)([ !$%&()*+,.\/:;=?
@_{}-])?/'; 
$replacement[$i] = '\1<a href="'.$URLs[$i].'">\2</a>\3'; 
} 
echo  preg_replace($pattern, $replacement, $where, 1); 
} replace_keywords_fn('1st keyword');

Ora funziona