我正在使用此代码提交一个帖子表单并上载一个图像，我想将其作为帖子缩略图。所有内容都提交得很完美，图像甚至可以上传，但我似乎不知道如何将图像附加到帖子并将其自动设置为帖子缩略图。任何帮助都将不胜感激。

<?php

$postContentError = \'\';

if ( isset( $_POST[\'submitted\'] ) && isset( $_POST[\'post_nonce_field\'] ) && wp_verify_nonce( $_POST[\'post_nonce_field\'], \'post_nonce\' ) ) {

if ( trim( $_POST[\'postContent\'] ) === \'\' ) {
    $postContentError = \'Please enter a description of this property.\';
    $hasError = true;
}

$random = rand(1000000, 9999999);

$post_information = array(      
    \'post_name\' => sanitize_title( date( \'YmdHis\' ). \'-\' . $random),
    \'post_title\' => \'Property \'.date( \'YmdHis\' ). \'-\' . $random,
    \'post_content\' => $_POST[\'postContent\'],
    \'post_type\' => \'properties\',
    \'post_status\' => \'publish\'
);

$propertyfor = $_POST[\'propertyfor\'];
$propertytype = $_POST[\'propertytype\'];
$bedrooms = $_POST[\'bedrooms\'];

$post_id = wp_insert_post($post_information);

function upload_user_file( $file = array() ) {
      require_once( ABSPATH . \'wp-admin/includes/admin.php\' );
      $file_return = wp_handle_upload( $file, array(\'test_form\' => false ) );
      if( isset( $file_return[\'error\'] ) || isset( $file_return[\'upload_error_handler\'] ) ) {
          return false;
      } else {
          $filename = $file_return[\'file\'];
          $attachment = array(
              \'post_mime_type\' => $file_return[\'type\'],
              \'post_title\' => preg_replace( \'/\\.[^.]+$/\', \'\', basename( $filename ) ),
              \'post_content\' => \'\',
              \'post_status\' => \'inherit\',
              \'guid\' => $file_return[\'url\']
          );
          $attachment_id = wp_insert_attachment( $attachment, $file_return[\'url\'], $post_id );
          require_once(ABSPATH . \'wp-admin/includes/image.php\');
          $attachment_data = wp_generate_attachment_metadata( $attachment_id, $filename );
          wp_update_attachment_metadata( $attachment_id, $attachment_data );
          if( 0 < intval( $attachment_id ) ) {
                return $attachment_id;
          }
      }
      return false;
}

if( ! empty( $_FILES ) ) {
  foreach( $_FILES as $file ) {
    if( is_array( $file ) ) {
      $attachment_id = upload_user_file( $file );
    }
  }
}

if($post_id) {
    // Update Custom Meta
    update_post_meta($post_id, \'shru_price\', esc_attr(strip_tags($_POST[\'shru_price\'])));
    update_post_meta($post_id, \'shru_address\', esc_attr(strip_tags($_POST[\'shru_address\'])));

    wp_set_object_terms( $post_id, $propertyfor, \'propertyfor\' );
    wp_set_object_terms( $post_id, $propertytype, \'propertytype\' );
    wp_set_object_terms( $post_id, $bedrooms, \'bedrooms\' );

    // Redirect
    wp_redirect(home_url(\'/listings\'));
    exit;
}

}

?>