How to add a predefined custom field without using a plugin?

Question

I´m trying to find a snippet to add a predefined custom field. Does anyone know how or where i can find this?

I know I can do this with CustomPress, but I´m trying to do this without a plugin.

Answer 1

Take a look at adding a custom meta box for any type of predefined meta field.

add_meta_box - Docs - WordPress

You will need to add a hook to the 'save_post' action in order to save the meta data also.

Here is a quick and simple version:

    add_action( 'save_post', 'your_meta_box_save' );

    add_action( 'add_meta_boxes', 'your_meta_box_add' );

function your_meta_box_add(){
        add_meta_box( 'predefined_field', 'Your Predefined Info', 'your_meta_box_html', 'post', 'normal', 'high' );
}

function your_meta_box_save( $post_id ){

    // Bail if we're doing an auto save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    // if our nonce isn't there, or we can't verify it, bail
    if( !isset( $_POST['meta_box_nonce'] ) || !wp_verify_nonce( $_POST['meta_box_nonce'], 'your_meta_box_nonce' ) ) return;

    // if our current user can't edit this post, bail
    if( !current_user_can( 'edit_post' ) ) return;

    // now we can actually save the data
    $allowed = array( 
        'a' => array( // on allow a tags
            'href' => array() // and those anchords can only have href attribute
        )
    );

    $your_predefined_value = isset($_POST['your_predefined_field']) ? $_POST['your_predefined_field'] : '';

    if( $your_predefined_value )
        update_post_meta($post_id,'your_predefined_field',$your_predefined_value);

}

function your_meta_box_html( $post ){

    wp_nonce_field( 'your_meta_box_nonce', 'meta_box_nonce' );

    //if you know it is not an array, use true as the third parameter
    $your_predefined_value = get_custom_meta($post->ID,'your_predefined_field',true);

    ?>

            <input name="your_predefined_field" id="your_predefined_field" type="text"  value="<?php echo $your_predefined_value; ?>" class="mws-textinput" />
    <?php
}

That should get you started; comment back if something is amiss.