Custom WordPress Metabox

functions.php

add_action( 'add_meta_boxes', 'mg_mb_create' );

function mg_mb_create() {
    
    add_meta_box( 'mg-meta', 'My Custom Meta Box', 'mg_mb_function', 'post', 'normal', 'high'  );
}

function mg_mb_function( $post ) {
    
    // Retrieve the metadata values if they exist.
    $mg_mb_name = get_post_meta( $post->ID, '_mg_mb_name', true );
    $mg_mb_genre = get_post_meta( $post->ID, '_mg_mb_genre', true );
    
    echo 'Please fill out the information below';
    ?>

    <p>
        Name: 
        <input type="text" name="mg_mb_name" value="<?php echo esc_attr( $mg_mb_name ); ?>" />
    </p>
    <p>
        Genre: 
        <select name="mg_mb_genre">
            <option <value="rock" <?php selected( $mg_mb_genre, 'rock' ); ?>>
                Rock
            </option>
            <option <value="jazz" <?php selected( $mg_mb_genre, 'jazz' ); ?>>
                Jazz
            </option>
            <option <value="hip-hop" <?php selected( $mg_mb_genre, 'hip-hop' ); ?>>
                Hip-Hop
            </option>
        </select>
    </p>
    <?php
}

// Hook to save the metabox data
add_action( 'save_post', 'mg_mb_save_meta' );

function mg_mb_save_meta( $post_id ) {
    
    // Verify the metadata is set.
    if ( isset( $_POST['mg_mb_name'] ) ) {
        
        // Save the metadata.
        update_post_meta( $post_id, '_mg_mb_name', strip_tags( $_POST['mg_mb_name'] ) );
        update_post_meta( $post_id, '_mg_mb_genre', strip_tags( $_POST['mg_mb_genre'] ) );

    }
}