164 of crpc

Section 164, CrPC clarifies that for a statement to be valid as a confession, it can be made to a judicial magistrate even if he lacks the jurisdiction, but has to be made during the course of the investigation, before the inquiry or trial commences.

Moreover, section 26 of the Indian Evidence act essentially states that a statement made by a person in police custody can be used as proof against him as long as it is made in the presence of a magistrate. (Section 25 renders confessions made to police officers invalid, and section 26 provides an exception to section 25.) 

So, technically, a confession recorded by a magistrate while the accused is still in PCR could be valid. It must however meet other requirements of section 164, CrPC to be valid. (The confession has to be made voluntarily, etc.)