thinkphp随机有关问题

$arr = array( 
array('id' => 1,'name' => 'aaa','uid'=>1), 
array('id' => 2,'name' => 'bbb','uid'=>2), 
array('id' => 3,'name' => 'ccc','uid'=>3), 
array('id' => 4,'name' => 'ddd','uid'=>4), 
array('id' => 5,'name' => 'ccc','uid'=>4), 
array('id' => 6,'name' => 'bbb','uid'=>4), 
array('id' => 7,'name' => 'bbb','uid'=>5), 
array('id' => 8,'name' => 'fff','uid'=>6), 
array('id' => 9,'name' => 'ccc','uid'=>7), 
array('id' => 10,'name' => 'bbb','uid'=>8), 
array('id' => 11,'name' => 'ddd','uid'=>8), 
array('id' => 12,'name' => 'eee','uid'=>9), 
array('id' => 13,'name' => 'fff','uid'=>10), 
); 
usort($arr, function($a, $b) {
  if($a['uid'] == $b['uid']) return rand(-1, 1);
  return $a['uid'] > $b['uid'] ? 1 : -1;
  });
print_r($arr);