$json = <<<EOT
[
{"album":"album1","truck":"truck1"},
{"album":"album1","truck":"truck3"},
{"album":"album1","truck":"truck4"},
{"album":"album1","truck":"truck6"},
{"album":"album2","truck":"truck2"},
{"album":"album2","truck":"truck3"},
{"album":"album3","truck":"truck2"},
{"album":"album3","truck":"truck5"}
{"album":"album4","truck":"truck5"},
{"album":"album4","truck":"truck6"},
{"album":"album5","truck":"truck1"},
{"album":"album5","truck":"truck3"},
{"album":"album6","truck":"truck2"},
{"album":"album6","truck":"truck3"},
{"album":"album6","truck":"truck4"},
{"album":"album7","truck":"truck2"},
{"album":"album7","truck":"truck5"},
{"album":"album7","truck":"truck6"}
]
EOT;
$data = json_decode($json);
foreach($data as $row){
   //echo '唱片:'.$row->album.' ,曲目:'.$row->truck.'<br />';
}

$data = json_decode($json);
foreach($data as $v) $r[$v->truck] = $v;
$data = $r;
$ar = array_rand($data, 4);
foreach($ar as $k) {
    echo '唱片:'.$data[$k]->album.' ,曲目:'.$data[$k]->truck.'<br />';
}