Better test:

    time for f in `seq 1000` ; do php -r '$data=[1,2,3];';  done

real 0m11.169s user 0m6.765s sys 0m4.477s

    time for f in `seq 1000` ; do php -r '$data=json_decode("[1,2,3]");';  done

real 0m9.997s user 0m6.093s sys 0m3.974s