Apparently both frequency and phase are independent properties but what is the relation between frequency and phase in PLL?so that when one is locked ,other is also locked
no, frequency and phase are not independent quantities. phase is the integral of frequency. and in the VCO (or NCO) inside a PLL, there is an integrator. the frequency is proportional to the input of the VCO or NCO, but the phase (which is what goes into the phase discriminator) is the integral of that.
with feedback, a PLL is a servo-control mechanism. but because of the inherent integrator in the loop, the "controller" is either a PI or an I. not much of a D in that controller inside a PLL.