ftp - Restrict access of subdirectory PHP -

i have multiple directories in hosting directory. let's

dir_a dir_b dir_c 

i created ftp account each directory, user of each directory can't access other dirs ( user of dir_a can't access dir_b, etc.)

but realize php can run readdir , content of file/dir unless unix permissions restrict that. did search , found perhaps need set basedir in php.ini settings, don't know how. shared hosting account , can't modify system php.ini. can put php.ini or user.ini in subdirectory , solve problem ? don't have time learn php.ini @ moment please don't send me links learn ( when have time ). please provide example.

i understand php.ini, restrict access php files, guess that's enough if there no higher level flag.

thanks.