How to validate select field in post meta?

Question

I want to display a select dropdown field in the post meta. How can I validate the input before saving it?

Currently, I am simply saving without validating like this:

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

Here's the code I'm using to display the meta field:

$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>

Answer 1

Separate the allowed values in a function:

function get_allowed_values()
{
    return array('one', 'two', 'three');
}

In your metabox:

foreach ( get_allowed_values() as $item) {}

In your validation handler:

if ( isset( $_POST['select_meta'] ) 
    and in_array( $_POST['select_meta'], get_allowed_values() )
)