See Where PHP

I would like to know why this part of the code is not for me. I am testing using a base in phpMyAdmin in xampp.

$conexion=new PDO('mysql:host=localhost;dbname=personas',"root", "");

$consultaa = $conexion->prepare("SELECT * WHERE Cuil=:cuil")
$consultaa->execute(array(
    ':cuil' => $xcuil
));
if ($xcuil=mysql_fetch_array($consultaa)) {
    echo "<script> alert("Cuil ya Ingresado") </script>";
    exit();
}