How to check for email?

I'm inserting users into the database and clicking "submit" on the form says to insert the email and this email already exists.

I've been analyzing I changed [rowCount()>=0] to [rowCount()>=1] , gave

  

Internal error 1 while checking email

I mean, never validate the email, I'm grateful!

require_once( '../../site/classes/DB.class.php' );

if($_SERVER['REQUEST_METHOD'] == 'POST' ) {
  $email =     $_POST['email'];
  $erro = '';


 $verificar = DB::getConn()->prepare("SELECT 'id' FROM 'usuarios' WHERE 'email'=?");

  if( $verificar->execute( array( $email) ) ) {
     if( $verificar->rowCount()>=0){
        $erro .= 'Este e-mail ja existe<br>';
     } else {
        $erro .= 'Erro interno 1 ao verificar o e-mail<br>';
     }
  } else {
     $erro .= 'Erro interno 2 ao verificar o e-mail<br>';
  }

? >