rootfunctions.php

add_action('add_meta_boxes', 'alispx_meta_boxes');
function alispx_meta_boxes() {
	add_meta_box( 
		'alispx-meta-box', 
		__( 'Phone Model', THEMO_TEXT_DOMAIN ), 
		'alispx_meta_box_callback', 
		'page' 
	);
}

function alispx_meta_box_callback( $post ) {
	 
	wp_nonce_field( 'alispx_meta_box', 'alispx_meta_box_noncename' );

	$post_meta = get_post_meta( $post->ID, 's4root_phone_model', true );

	$current_value = '';
	
	if ( ! empty( $post_meta ) ) {
		$current_value = $post_meta;
	}
	 ?>
	 <div class="form-wrap">
		<div class="form-field" style="clear:both;">
			 <label class="label" for="s4root_phone_model">Phone Model</label>
			 <input  name="s4root_phone_model" id="s4root_phone_model" type="text" value="<?php echo esc_attr( $current_value ); ?>">
			<p>Enter the phone model</p>
		</div>
	</div>
	 <?php
}

add_action( 'save_post', 'alispx_save_custom_fields', 10, 2 );
function alispx_save_custom_fields( $post_id, $post ) {

	if ( 'page' != $post->post_type || ! current_user_can( 'edit_post', $post_id ) ) {
		return;
	}
		   
	if ( !isset( $_POST['alispx_meta_box_noncename'] ) || ! wp_verify_nonce( $_POST['alispx_meta_box_noncename'], 'alispx_meta_box' ) ) {
			return;
	}
			
	if( isset($_POST['s4root_phone_model']) && $_POST['s4root_phone_model'] != "" ) {
			update_post_meta( $post_id, 's4root_phone_model', sanitize_text_field( $_POST['s4root_phone_model'] ) );
	} else {
		if ( isset( $post_id ) ) {
			delete_post_meta($post_id, 's4root_phone_model');
		}
	}
	
	
}