How to set a featured image (thumbnail) with PHP?

Question

I am creating posts using wp_insert_post(), but these posts do not have featured images.

I have a post ID, and a path to the image on my server, how do I set them as the featured image?

EDIT : I added my code to publish an article. It only needs to add the thumbnail (featured image).

imagePath= ABSPATH . "wp-content\uploads\image\my_image.jpg";
publication($titre, $content, $categriesID, $tags);


function publication ($titre, $content, $categories, $tags){
    $user_id = get_current_user_id();

    $my_post = array(
        'post_title'    => $titre,
        'post_content'  => $content,
        'post_status'   => 'publish',
        'post_author'   => $user_id,
        'post_category' => $categories,
        'tags_input'      => $tags
     );

    $post_ID = wp_insert_post( $my_post );
}

Answer 1

I am proud to announce that I have found the solution thanks to @cjbj good advice. I paste the code here. I know it will help others.

In addition, I used the function preg_quote to protect the path to the image.

protected_path = preg_quote($filename);

This code works great.

// $filename should be the path to a file in the upload directory.
$filename = '/path/to/uploads/2013/03/filename.jpg';

// The ID of the post this attachment is for.
$parent_post_id = 37;

// Check the type of file. We'll use this as the 'post_mime_type'.
$filetype = wp_check_filetype( basename( $filename ), null );

// Get the path to the upload directory.
$wp_upload_dir = wp_upload_dir();

// Prepare an array of post data for the attachment.
$attachment = array(
    'guid'           => $wp_upload_dir['url'] . '/' . basename( $filename ), 
    'post_mime_type' => $filetype['type'],
    'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $filename ) ),
    'post_content'   => '',
    'post_status'    => 'inherit'
);

// Insert the attachment.
$attach_id = wp_insert_attachment( $attachment, $filename, $parent_post_id );

// Make sure that this file is included, as wp_generate_attachment_metadata() depends on it.
require_once( ABSPATH . 'wp-admin/includes/image.php' );

// Generate the metadata for the attachment, and update the database record.
$attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
wp_update_attachment_metadata( $attach_id, $attach_data );

set_post_thumbnail( $parent_post_id, $attach_id );