Create a very basic rest api

rest-api.php

<?php
/**
 * Plugin Name: Awesome API Test
 * Description: Awesome Desc...
 * Plugin URI:  #
 * Version:     1.0
 * Author:      #
 * Author URI:  http://github.com/anisur2805/
 * Text Domain: test-domain
 * License:     GPL v2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */

if ( !defined( 'ABSPATH' ) ) {
 exit;
}

define( 'WPPLUGIN_DIR', __DIR__ );

// function test_init(){
//     $api_url = 'https://api.github.com/users';
//     $args = [
//         'method'     => 'GET'
//     ];
//     $response = wp_remote_get($api_url, $args);

//     if( is_wp_error( $response ) ) {
//         $error_msg = $response->get_error_message();
//         return "Something went wrong: " . $error_msg;
//     }

//     $results = json_decode( wp_remote_retrieve_body( $response ) );

//     error_log( print_r( $results, true ) );

// }
// test_init();


// add_action( 'admin_menu', 'wpplugin_admin_menu' );

// function wpplugin_admin_menu() {
//  add_menu_page( 'Testing', 'Testing', 'manage_options', 'testing', 'testing_callback' );
// }

// function testing_callback() {
//  $url  = 'https://jsonplaceholder.typicode1.com/users';
//  $args = [
//   'method' => 'GET',
//  ];

//  $response = wp_remote_get( $url, $args );

//  if ( 200 == wp_remote_retrieve_response_code( $response ) ) {
//   $file_link = WPPLUGIN_DIR . '/query-apis/data.json';
//   $message   = wp_remote_retrieve_body( $response );

//   write_to_file( $message, $file_link );

//  }

//  if ( is_wp_error( $response ) ) {
//   $file_link = WPPLUGIN_DIR . '/query-apis/error.txt';
//   $error_msg = $response->get_error_message();
//   $msg       = date( 'd m Y g:i:a' ) . ' ' . wp_remote_retrieve_response_code( $response ) . ' ' . $error_msg;

//   write_to_file( $msg, $file_link );

//  }
// }

// function write_to_file( $message, $file_link ) {
//  if ( file_exists( $file_link ) ) {
//   $filling = fopen( $file_link, 'a' );
//   fwrite( $filling, $message . "\n" );
//  } else {
//   $filling = fopen( $file_link, 'w' );
//   fwrite( $filling, $message, "\n" );
//  }
//  fclose( $filling );
// }

// create a rest api endpoint
add_action( 'rest_api_init', 'register_rest_api_init' );
function register_rest_api_init() {
 register_rest_route(
  'ar/v1/',
  'test-callback',
  array(
   array(
    'methods'  => 'POST',
    'callback' => 'ar_post_callback',
   ),
   array(
    'methods'  => 'GET',
    'callback' => 'ar_get_callback',
   ),
  )
 );

 register_rest_field(
  'post',
  'feat_img',
  array(
   'get_callback'    => 'rest_api_feature_image_callback',
   'update_callback' => null,
   'schema'          => null,
  )
 );

 register_rest_field(
  'post',
  'author',
  array(
   'get_callback'    => 'rest_api_author_callback',
   'update_callback' => null,
   'schema'          => null,
  )
 );

 register_rest_field(
  'post',
  'author_description',
  array(
   'get_callback'    => 'rest_api_author_description_callback',
   'update_callback' => null,
   'schema'          => null,
  )
 );

}

function rest_api_feature_image_callback( $object ) {
 $image_src = wp_get_attachment_image_src( $object['featured_media'], 'medium' );
 return $image_src;
}

function rest_api_author_callback() {
 $author = get_the_author_meta( 'display_name' );
 return $author;
}

function rest_api_author_description_callback( $object ) {
 $author_description = get_the_author_meta( 'user_description' );
 return $author_description;
}

function ar_post_callback( $request_params ) {
 $data = array();

 $params = $request_params->get_params();

 $name     = $params['name'];
 $password = $params['password'];
 $title    = $params['title'];
 $content  = $params['content'];

 if ( isset( $name ) && isset( $password ) ) {

  $userData = get_user_by( 'login', $name );
  if ( $userData ) {
   $wp_check_password_result = wp_check_password( $password, $userData->user_pass, $userData->ID );

   if ( $wp_check_password_result ) {
    $data['received_data'] = array(
     'name'     => $name,
     'password' => $password,
     //  'data'     => $userData
    );

    $post_args = array(
     'post_title'   => wp_strip_all_tags( $title ),
     'post_content' => $content,
     'post_status'  => 'publish',
     'post_type'    => 'apiData',
    );

    $inserted_id = wp_insert_post( $post_args );

    if ( $inserted_id ) {

     $data['status']  = 'OK';
     $data['message'] = __( 'Alhamdulillah, ' ) . $title . ' was created successfully.';

    } else {

     $data['status']  = 'Insert Failed';
     $data['message'] = __( 'Innalillah, post insert failed' );
    }

   } else {
    $data['status']  = 'OK';
    $data['message'] = __( 'You are not authenticated' );
   }
  } else {
   $data['status']  = 'OK';
   $data['message'] = __( 'User dose not exits' );
  }

 } else {
  $data['status']  = 'Failed';
  $data['message'] = __( 'Innnalillah' );
 }

 return $data;
}

function ar_get_callback( $request_params ) {
 $data = array();

 $params = $request_params->get_params();

 $name     = $params['name'];
 $password = $params['password'];

 $data = [
  'status'   => 'OK',
  'name'     => $name,
  'password' => $password,
 ];

 return $data;
}

// register a custom post type
function my_register_post_type() {
 $args = array(
  'public'             => true,
  'publicly_queryable' => false,
  'label'              => __( 'Rest API' ),
  'supports'           => array( 'title', 'editor' ),
 );

 register_post_type( 'apiData', $args );

}

add_action( 'init', 'my_register_post_type' );