Get First Post content and edit it using pre_get_posts (or similar?!)

Question

I have a custom plugin, that creates an admin settings page, with 3 fields:

postIDs, pageIDs and Message.

I am trying to grab the first post displayed on the homepage list and add the contents of the field "message" to the beginning of the content. I'm doing this within a plugin, and I can't work out how to hook into the homepage posts list, get the first post there, and add the "Message" before the content then return it back.

I figured it was something to do with pre_get_posts hook - but I just can't work out the correct way to do it.

Here's the full PHP of the plugin, it all works, I just can't now work out the correct way to get the first post content :(

<?php
/*
Plugin Name: Custom HTML
Description: Custom HTML in Pages
Version: 1.0.0
*/
class Codeable_Fields_Plugin {
  public function __construct() {
    // Hook into the admin menu
    add_action( 'admin_menu', array( $this, 'create_plugin_settings_page' ) );
    // Add Settings and Fields
    add_action( 'admin_init', array( $this, 'setup_sections' ) );
    add_action( 'admin_init', array( $this, 'setup_fields' ) );
  }
  public function create_plugin_settings_page() {
    // Add the menu item and page
    $page_title = 'Custom HTML';
    $menu_title = 'Custom HTML';
    $capability = 'manage_options';
    $slug = 'codeable_fields';
    $callback = array( $this, 'plugin_settings_page_content' );
    $position = 100;
    add_menu_page( $page_title, $menu_title, $capability, $slug, $callback, $icon, $position );
  }
  public function plugin_settings_page_content() {?>
    <div class="wrap">
      <?php
      if ( isset( $_GET['settings-updated'] ) && $_GET['settings-updated'] ){
        $this->admin_notice();
      } ?>
      <form method="POST" action="options.php">
      <?php
        settings_fields( 'codeable_fields' );
        do_settings_sections( 'codeable_fields' );
        submit_button();
      ?>
      </form>
    </div> <?php
  }
  public function admin_notice() { ?>
    <div class="notice notice-success is-dismissible">
      <p>Your settings have been updated!</p>
    </div><?php
  }
  public function setup_sections() {
    add_settings_section( 'our_first_section', 'Custom HTML', array( $this, 'section_callback' ), 'codeable_fields' );
  }
  public function section_callback( $arguments ) {
    switch( $arguments['id'] ){
      case 'our_first_section':
        echo '';
        break;
      }
  }
  public function setup_fields() {
    $fields = array(
      array(
        'uid' => 'codeable_pages_field',
        'label' => 'Page IDs to Exclude',
        'section' => 'our_first_section',
        'type' => 'text',
        'supplimental' => 'Seperate by comma',
      ),
      array(
        'uid' => 'codeable_posts_field',
        'label' => 'Post IDs to Exclude',
        'section' => 'our_first_section',
        'type' => 'text',
        'supplimental' => 'Seperate by comma',
      ),
      array(
        'uid' => 'codeable_textarea',
        'label' => 'Enter Text here',
        'section' => 'our_first_section',
        'type' => 'textarea',
      )
    );
    foreach( $fields as $field ){
      add_settings_field( $field['uid'], $field['label'], array( $this, 'field_callback' ), 'codeable_fields', $field['section'], $field );
      register_setting( 'codeable_fields', $field['uid'] );
    }
  }
  public function field_callback( $arguments ) {
    $value = get_option( $arguments['uid'] );
    if( ! $value ) {
      $value = $arguments['default'];
    }
    switch( $arguments['type'] ){
      case 'text':
      case 'password':
      case 'number':
        printf( '<input name="%1$s" id="%1$s" type="%2$s" placeholder="%3$s" value="%4$s" />', $arguments['uid'], $arguments['type'], $arguments['placeholder'], $value );
        break;
      case 'textarea':
        printf( '<textarea name="%1$s" id="%1$s" placeholder="%2$s" rows="5" cols="50">%3$s</textarea>', $arguments['uid'], $arguments['placeholder'], $value );
        break;
      case 'select':
      case 'multiselect':
        if( ! empty ( $arguments['options'] ) && is_array( $arguments['options'] ) ){
          $attributes = '';
          $options_markup = '';
          foreach( $arguments['options'] as $key => $label ){
            $options_markup .= sprintf( '<option value="%s" %s>%s</option>', $key, selected( $value[ array_search( $key, $value, true ) ], $key, false ), $label );
          }
          if( $arguments['type'] === 'multiselect' ){
            $attributes = ' multiple="multiple" ';
          }
          printf( '<select name="%1$s[]" id="%1$s" %2$s>%3$s</select>', $arguments['uid'], $attributes, $options_markup );
        }
        break;
      case 'radio':
      case 'checkbox':
        if( ! empty ( $arguments['options'] ) && is_array( $arguments['options'] ) ){
          $options_markup = '';
          $iterator = 0;
          foreach( $arguments['options'] as $key => $label ){
            $iterator++;
            $options_markup .= sprintf( '<label for="%1$s_%6$s"><input id="%1$s_%6$s" name="%1$s[]" type="%2$s" value="%3$s" %4$s /> %5$s</label><br/>', $arguments['uid'], $arguments['type'], $key, checked( $value[ array_search( $key, $value, true ) ], $key, false ), $label, $iterator );
          }
          printf( '<fieldset>%s</fieldset>', $options_markup );
        }
        break;
      }
      if( $helper = $arguments['helper'] ){
        printf( '<span class="helper"> %s</span>', $helper );
      }
      if( $supplimental = $arguments['supplimental'] ){
        printf( '<p class="description">%s</p>', $supplimental );
      }
    }
  }
}
new Codeable_Fields_Plugin();

add_action ('the_content', 'add_to_header');
function add_to_header(){
  $postids = preg_split("/\s*,\s*/", get_option('codeable_posts_field'));
  $pageids = preg_split("/\s*,\s*/", get_option('codeable_pages_field'));

  if (is_single($postids )) {
    $fullcontent = $content;
  }
  elseif (is_page($pageids)) {
    $fullcontent = $content;
  }
  elseif (is_single() || is_page()) {
    $beforecontent = get_option('codeable_textarea');
    $fullcontent = $beforecontent . $content;
  }
  return $fullcontent;
}

Any help would be gratefully received!!

Answer 1

You can use the loop_start hook which passes the $WP_Query object by reference.

namespace StackExchange\WordPress;
class editFirstPostContent {
  public function load() {
    if( \is_admin() ) {
      return;
    }
    \add_filter( 'loop_start', [ $this, 'loopStart' ] );
  }
  public function loopStart( \WP_Query $WP_Query) {
    if( ! $WP_Query->is_main_query() ) {
      return;
    }
    if( ! isset( $WP_Query->posts[0] ) ) {
      return;
    }
    $WP_Query->posts[0]->post_content = sprintf(
      '<div>Hello World!</div>%1$s',
      $WP_Query->posts[0]->post_content
    );
  }
}
$editFirstPostContent = new editFirstPostContent();
\add_action( 'plugins_loaded', [ $editFirstPostContent, 'load' ] );

Answer 2

For reference, I solved it with this code:

function is_latest() {
    global $post;
    $loop = get_posts( 'numberposts=1' );
    $latest = $loop[0]->ID; 
    return ( $post->ID == $latest ) ? true : false;
}   

Then called the is_latest() function into the if statement at the end