Can post-doc researchers write a recommendation letter for someone applying for a PhD program? I am wondering how it is treated if a post-doc researcher writes a recommendation letter for someone applying to get PhD admission.

Will the selection committee evaluate it properly?
Academic rank varies by country.  But I would expect in most places a letter written by a postdoc would be considered.  Selecting letter writers is often a compromise between getting a letter writer who is highly ranked and well known and getting a letter writer who is well informed.  Typically one gets both when a postdoc assists a senior faculty member in writing a letter.