PHP noob, creating registration system error -

i attempting write user registration system web site. running php 5 on web server , getting error:

parse error: syntax error, unexpected ';' in /nfs/.../processreg.php on line 18

line 18: if (mysql_num_rows($s?>0)) 

the rest of code this:

<?php  include("db.php");  if (isset($_post['username']) && isset($_post['password']) && isset($_post['email'])) {     //prevent sql injections     $username = mysql_real_escape_string($_post['username']);     $email    = mysql_real_escape_string($_post['email']);      //get md5 hash of password     $password = md5($_post['password']);      //check see if username exists     $sql = mysql_query("select username usersystem username = '".$username."'");     if(mysql_num_rows($s?>0)) {         die("username taken.");     }      mysql_query("insert usersystem (username, password, email) values ( '$username', '$password', '$email')") or die (mysql_error()); echo "account created."); }  ?> 

i not understand error because there not ';' @ end of line.

if (mysql_num_rows($s?>0)) 

should be

if (mysql_num_rows($sql) > 0) 

and....

die (mysql_error()); echo "account created.";) 

should

die (mysql_error()); echo "account created.";