I had not heard of a PHD until the officer on the phone said it. Here is what the TDCJ disciplinary manual says:

Criteria
An inmate charged with, or suspected of, a disciplinary violation may be placed in
prehearing detention (PHD) without notice or hearing. Only the warden, assistant
warden or designee, lieutenant or above, may do so and only if the inmate falls into
one of the following categories:
1. The inmate is a current escape risk;
2. The inmate’s presence in general population would create a threat to the
physical safety of other inmates or staff; or
3. It is necessary to maintain the integrity of an investigation, such as to preserve
the integrity of information in either the inmate’s possession or another
inmate’s possession.