Problems using a PHP string variable in a MySQL select query

I've found many similar questions regarding this but anything I try won't work.

I'm trying to run a MySQL query using the variable $epost. When I echo this variable it displays correctly, but the query retus nothing. Entering a fixed value for $epost like:

$epost='email@email.com'

Retus the correct query from the database.

$epost=mysqli_real_escape_string($co,$_POST['email']);
  echo $epost;

$sql = "SELECT memberID FROM Member WHERE email = '$epost' limit 1";
$result = mysqli_query($co,$sql);
$row = mysqli_fetch_assoc($result);
echo $row["memberID"];