Trying to to add a string of text between an empty <a href></a> tag.

Tried this code, but it doesn't seem to work.
I have serached and searched the net for a solution, but I'm guessing I don't know the correct keywords for searching.

Closest I seem to have come is this code.

PHP Code:
<?php
      $string 
'<img src="images/blank.gif" class="linklayer" />';
      
preg_replace('\/<a>([^>]*)<\/a>/is'$string);
      
      echo 
$string;
?>
As anyone knowing how to program I don't know how o handle my variables.

All that happens is that the value of string is echoed in the beginning of the document, not in between the tags i want them.

Suggestions anyone?

Thanks in advance.