Problem with preg_replace

Currently, I am parseing a string for href tags and adding a "target=_blank" to them to have them open up in a new window using this: $body = preg_replace("<[aA] [hH][rR][eE][fF]=\"(.*?)\">", 'a href="$1" target=_blank', $row->body); This works, to the best of my knowledge, however I was curious if I could have the target generated based on the url, so that each url would open in its own window. I decided to try to use the url's md5 to be its target, and assumed that this would work: $body = preg_replace("<[aA] [hH][rR][eE][fF]=\"(.*?)\">", 'a href="$1" target=' . md5($1), $row->body); however, I am getting this as an error: Parse error: parse error, expecting T_VARIABLE' or '$'' in index.php on line 201 Can anyone see what is going wrong here?