you could always build from source
the management is not keen to upgrade RHN subscriptions.
You do at least HAVE a long term support contract , for the backported security fixes , right ?
this copy of 5.3 is a registered copy ?
red hat is not free you MUST have a longterm ( 5 to 7 year for the older 5.3) support contract
for xen ? rhel6.1 is much better but it will be in rhn
yum search xen
---- and ----
yum search phpmyadmin
should show results
but for phpomyadmin i would grab it from there web site and manually install