我想在post meta中显示一个select下拉字段。如何在保存之前验证输入？

目前，我只是简单地保存而不进行验证，如下所示：

if ( isset( $_POST[\'select_meta\'] ) ) {     
        update_post_meta( $post->ID, \'select_meta\', $_POST[\'select_meta\'] );    
    }

$select_items = (\'one\', \'two\', \'three\');

$select_meta = get_post_meta( $post->ID, \'select_meta\', true ); ?>

<select id="select_meta" name="select_meta">
    <?php
        if($select_items){
            foreach ($select_items as $item){?>
                <option <?php selected( $item == $select_meta ); ?> value="<?php echo $item; ?>"><?php echo $item ?></option>                           
                <?php                   
            }
        }
    ?>      
</select>