对于唯一的4位数，以下代码可以正常工作

function unique_number($length=10) {

   $string = \'\';
   // You can define your own characters here.
   $characters = "0123456789";

   for ($p = 0; $p < $length; $p++) {
       $string .= $characters[mt_rand(0, strlen($characters)-1)];
   }

   return $string;
}

print unique_number(4);

我给大家举一个ACF自定义字段的例子。

//for acf
function my_acf_load_field( $field ) {

     $rand = unique_number(4);
     $field[\'default_value\'] = $rand;
     return $field;

}

add_filter(\'acf/load_field/name=your_field_name\', \'my_acf_load_field\');