$string = 'last , first';
list($last,$first) = preg_split("/\s+,\s+/",$string);
$s = preg_split("/[, ]/",$string);
print implode(" ", array($s[0], end($s)));
print implode(" ", array($last,$first));
print preg_replace("/(\w+)\s+,\s+(\w+)/","$2 $1", $string);