<?php

$json =  <<<EOT
[{
    "postid": 66,
    "values": [
        "field_key=a",
        "oldvalue=b",
        "newvalue=c dad"
    ]
}]
EOT;

var_dump(json_decode($json));

?>

array(1) {
  [0]=>
  object(stdClass)#1 (2) {
    ["postid"]=>
    int(66)
    ["values"]=>
    array(3) {
      [0]=>
      string(11) "field_key=a"
      [1]=>
      string(10) "oldvalue=b"
      [2]=>
      string(14) "newvalue=c dad"
    }
  }
}