json_decode($json, true) true什么意思

<?php
$json = '{"a":1,"b":2,"c":3,"d":4,"e":5}';

var_dump(json_decode($json));
var_dump(json_decode($json, true));

?>

object(stdClass)[1]
public 'a' => int 1
public 'b' => int 2
public 'c' => int 3
public 'd' => int 4
public 'e' => int 5

array (size=5)
'a' => int 1
'b' => int 2
'c' => int 3
'd' => int 4
'e' => int 5