Register custom meta boxes

custom-meta-box.php

// Register a meta box under the services section

function service_meta_box() {
    add_meta_box(
        'service_icon',//id
        'Service Metabox',//title
        'service_icon_callback',//callback
        'service',//screen
        'side',//context
        'high'//priority
    ); 
    
    // $screen = array('post' , 'team' , 'page' , 'service');
    
}
add_action('add_meta_boxes','service_meta_box');


function service_icon_callback($post) {
    
        //input type value call as a variable (input box name is "s_icon")
        $s_icon = get_post_meta($post->ID,'s_icon',true);

        //wp_nonce_field('action','name', '$referer', '$echo');
        //The nonce field is used to validate that the contents of the form request came from the current site and not somewhere else.
        wp_nonce_field('save_service_meta','service_nonce');

    ?>
   
    <p>
        <label for="">Icon</label>
        <input type="text" name="s_icon" value="<?php echo $s_icon; ?>">
    </p>

    <?php 
}


function save_service_meta($post_id) {
    
    // check if our nonce is set.
    if(! isset($_POST ['service_nonce'])){
        return;
    }
    
    // verify that the nonce is valid.
    if (! wp_verify_nonce($_POST['service_nonce'], 'save_service_meta')) {
        return;
    }
    
    // make sure that the input box is set.
    if (! isset($_POST['s_icon'])) {
        return; 
    }
    
    // sanitize user input
    $my_s_icon = sanitize_text_field($_POST['s_icon']);
    
    // update the meta filed in the database.
    update_post_meta($post_id, 's_icon' , $my_s_icon);
    
}
add_action('save_post','save_service_meta');

metabox_info.txt

There are three steps of working metabox:
=========================================

1. First Function ===> Add Meta Box
2. Second Function ===> Callback ==> HTML Rendering
3. Third Function ===> Save Meta Box Data


1. Meta Box Add

2. Data Rendering

3. Data save process:

    3.1 ==> Nonce set
    3.2 ==> Nonce validation check
    3.3 ==> Confirm nonce option
    3.4 ==> Sanitize Data
    3.5 ==> Update Data with Id
    
4. Usage of Custom Meta Box:
==============================

<div class="icon">
    <i class="fa fa-<?php echo get_post_meta($post->ID,'s_icon',true); ?>"></i>
</div>