准备好的mysqli声明

我最近从mysql转到了mysqli，现在正试图将代码重新格式化为mysqli。

请告诉我这些说法有什么不对。

代码：

<?php
include_once 'dbconnect.php';
$sample ='sample123'; // these are dummy values
$orecip='admin';      // these are dummy values
$omessage='test';      // these are dummy values
$date = date('Y-m-d'); // these are dummy values
$read='yes';          // these are dummy values
$read1='no';          // these are dummy values
$otitle='test';       // these are dummy values
$insert = $MySQLi_CON->query("INSERT INTO messages (user_from, user_to, title, message, date, user_from_read, user_to_read)
    VALUES($sample, $orecip, $otitle ,$omessage,$date,$read,$read1)");
print '<h3>Insert Multiple records</h3>';
if($insert){
    //return total inserted records using mysqli_affected_rows
    print 'Success! Total ' .$MySQLi_CON->affected_rows .' rows added.<br />'; 
}else{
    die('Error : ('. $MySQLi_CON->errno .') '. $MySQLi_CON->error);
}
    ?>

错误：

错误：（1054）"字段列表"中的未知列"sample123"