php memory for copying arrays

"initial"
  1 => 17152320 - memory_get_usage()
  2 => 2097152  - memory_get_usage(true)
  3 => null     - delta for memory_get_usage()
  4 => null     - delta for memory_get_usage(true)

"file get, set to $str"
  1 => 25893440
  2 => 10485760
  3 => 8741120
  4 => 8388608

"explode $str to $arr"
  1 => 50129952
  2 => 23068672
  3 => 32977632
  4 => 20971520

"unset $str"
  1 => 41758984
  2 => 14680064
  3 => -8370968
  4 => -8388608

"$arr2 = $arr"
  1 => 41776648
  2 => 14680064
  3 => 17664
  4 => 0

"change $arr2[0]"
  1 => 52280152
  2 => 25165824
  3 => 10503504
  4 => 10485760

"change $arr[0]"
  1 => 52297736
  2 => 25165824
  3 => 17584
  4 => 0