反序列化josn为数组。

josn如下：
a:10:{
i:1;a:2:{s:8:"nickname";s:12:"MjEzMTLllYo=";s:10:"user_grade";i:7300;}
i:2;a:2:{s:8:"nickname";s:16:"6b6Z6JmO6L+Q5rCU";s:10:"user_grade";i:7200;}
i:3;a:2:{s:8:"nickname";s:8:"576K5a2Q";s:10:"user_grade";i:7000;}
i:4;a:2:{s:8:"nickname";s:8:"54uC54u8";s:10:"user_grade";i:6700;}
i:5;a:2:{s:8:"nickname";s:20:"4oC76KGA77+g5aSc4oC7";s:10:"user_grade";i:6700;}
i:6;a:2:{s:8:"nickname";s:4:"546l";s:10:"user_grade";i:6700;}
i:7;a:2:{s:8:"nickname";s:12:"6Z2S6b6Z5YmR";s:10:"user_grade";i:6600;}
i:8;a:2:{s:8:"nickname";s:8:"6Iql5YWw";s:10:"user_grade";i:6600;}
i:9;a:2:{s:8:"nickname";s:16:"5peg5pWM5bCK6ICF";s:10:"user_grade";i:6600;}
i:10;a:2:{s:8:"nickname";s:12:"5LiA6Iis6Iis";s:10:"user_grade";i:6500;}
}
 
怎样把该josn反序列化为数组。