$STH = $DBH->prepare_cached("SELECT x from y WHERE z=?");
$STH->execute('33'); 
$STH->fetch...; 
# Abfrage erst an Cache, dann DB

$STH = $DBH->prepare("SELECT x from y WHERE z=?");
$STH->execute('33'); 
$STH->fetch...; 
# Abfrage ohne Umschweife an DB