You're testing for description being logically true and longdescription matching whatever $search looks like. To get a case-insensitive match, you could transform everything to one case with UPPER(), say. Use placeholders.

$sth = $dbh->prepare(
    "SELECT * FROM `items`
    WHERE
      UPPER(description) LIKE UPPER(?)
    OR
      UPPER(longdescription) LIKE UPPER(?)");
$sth->execute($search,$search) or die $dbh->errstr;
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You should consider listing the columns you want, instead of using the wildcard. That way you don't fetch unneeded data, and can control the order of results.

After Compline,
Zaxo