PHP 数组函数 array_keys()

<?php
   $input = array("a"=>"Monkey","b"=>"Cat","c"=>"Dog");
   print_r(array_keys($input));

   $input = array("a"=>"Monkey","b"=>"Cat","c"=>"Dog");
   print_r(array_keys($input,"Dog"));

   $input = array(10,20,30,"10");
   print_r(array_keys($input,"10",false));
?>