Código PHP:
$mail_query = sprintf("SELECT * FROM users WHERE mail='%s'", //Check if the mail is used yet
mysql_real_escape_string($username));
if ($mail_ok=mysql_fetch_array($email)){
$confirm = 'El email introducido está en uso.';
}else{
require_once('recaptchalib.php');
Warning: mysql_fetch_array() expects parameter 1 to be resource, string given in /opt/lampp/htdocs/random/register.php on line 55