Hollow metal pocket door frames can really be the best choice when you're choosing what kind of door would be the most appropriate for your entrance. These types of hollow metal doors are great since they are very durable and strong, however they are not as heavy as fully filled door frames would be. With this construction you get reliability, strength, a good price as well as a door that isn't too heavy to shut and open.
There are several different kinds of companies that make this type of door such as Reliabilt, and they are often available in various sizes in order to accommodate different structures and homes. A metal pocket door is to prefer over wood in some cases: if you're concerned about warping and durability, then metals such as steel are very reliable and would be a good choice for any entrance.
ReliaBilt 32" x 80" Pocket Door Frame with Hardware
"For installation of pocket or bi parting doors up to 36" wide 1" to 1-3/8" thick panels weighing up to 150lbs. eachand up to 6ft 8in high. Heavy gauge, steel stiffener reinforces split jambs. Four large 1 In. wheels for easy rolling action. Snap in track and hanger system"
This unit is available at lowes.com
You could really use a unit like this for a selection of different doors, however the principle design is nice: hollow metal frames which add structure and durability to a door. Then it doesn't much matter what you cover the door with: however wood is of course a safe card if you care for classic design and charm. Wood however has to be maintained to some degree, but that isn't really a big deal, and the results can be very nice.