PHR is a MAC control element that the UE uses to tell the eNB, how much power it has been left with currently. Pmax - P_pusch . (Pmax is sent in SIB2)
RRC conn setup or RRC reconfig provides a MAC IE called phr-config which contains params that define the triggers for the UE to send PHR.
An interval for which the UE sends the PHR regularly.
A throttle timer which the UE should maintain between sending 2 consecutive PHRs. This value will be chosen to consider the round-trip delay
This is a threshold value which the DL-Pathloss (UE calculates this as a diff bw Signalled RS transmit power and the RSRP seen by the UE), cannot exceed. The moment this occurs, a PHR is sent.
In the PHR MAC CE, there are 6 bits that hold the PH value ranging from 0-63(PH0 means <=-22db and >=-23db, PH1 means -22to-21 and so on PH63 means >=40db)