$text = "Lorem ipsum dolor sit amet, consectetuer adipiscing elit,  :galerie=8: iustot nulla facilisi.";
function gall($subcat) {
         $bb_galeriecount_res = mysql_query("SELECT * FROM gal_pics WHERE subcat_id = '".$subcat."' ORDER BY rand() LIMIT 1");
         $bb_galeriecount = mysql_fetch_object($bb_galeriecount_res);
         $bilder = ''.$bb_galeriecount->subcat_id.' <b>'.stripslashes($subcat).'</b>';

         return $bilder;
}
$text = preg_replace("#\:galerie=(.*?)\:#si", gall("\\$1")."", $text);

echo $text;