WordPress - Uploading images to Advanced Custom Fields (AFC)

class-acf-image-upload.php

<?php
class ACF_Image_Upload {

    public $post_id;
    
    function update_acf ( ) {
		
        $attach_id = $this->get_image_attach_id('my-image.jpg');

        // Saving image
        update_field('image', $attach_id, $this->post_id);

    }
  	
    function get_image_attach_id ( $filename ) {
			
        // Get the path to the upload directory. 
        // If it was uploaded to WP, wp_upload_dir() does the job
        $wp_upload_dir = wp_upload_dir();
        $full_path = $wp_upload_dir['path'] . $filename;

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

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

        // Insert the attachment.
        $attach_id = wp_insert_attachment( $attachment, $full_path, $this->post_id );

        return $attach_id;
    }

}