Use `str_replace` to create a link

I need some help. I have a section that displays keywords "tags":

<?=str_replace(",",",",$line["m_tags"])?>

The code above looks like this

Tags: KEYWORD1, KEYWORD2, KEYWORD3

All I'm trying to do is have each individual KEYWORD be a hyperlink to link back to the main page. Any help would be appreciated.

The code you posted does nothing, it replaces , with ,. You can do this with regular expressions, but here is a different method:

$output = '';
$tmp    = explode(",",$line['m_tags']); /* convert to array */

foreach($tmp as $tag)
    $output .= '<a href="index.php">'.$tag.'</a>, '; /* put link in output */

echo substr($output,0,-2); /* echo output without the last , */

Shorter alternative as Felix Kling pointed out:

$tmp    = explode(",",$line['m_tags']); /* convert to array */

foreach($tmp as $key => $tag)
    $tmp[$key] = '<a href="index.php">'.$tag.'</a>'; /* put link back in tmp */

echo implode(",",$tmp);

Either this should work:

Tags: <? 
// php5.3
$tags=explode(",", $line["m_tags"]);
$tags = array_map(function($tag){
  return "<a href='http://www.yoursite.com/?tag=$tag'>$tag</a>";
}, $tags);
echo implode(", ", $tags);
?>

Here is how I would probably do it.

$str = "KEYWORD1,KEYWORD2,KEYWORD3";
$keywords = explode(',', $str);
$links = array();
foreach($keywords as $keyword) {
    $links[] = "<a href='home'>$keyword</a>";
}

echo implode(', ', $links);