WP Utility functions to get youtube/vimeo images and store their thumbnails as attachments

wp-video-thumbnail-attachments.php

<?php
function video_get_service( $video_url ) {
	if ( stristr( $video_url, 'vimeo.com') ) {
		return 'vimeo';
	}

	if ( stristr( $video_url, 'youtube.com' ) || stristr( $video_url, 'youtu.be' ) ) {
		return 'youtube';
	}

	return false;
}

function youtube_get_video_id( $video_url ) {
	if ( preg_match( '/(?:https?:\/{2})?(?:w{3}\.)?youtu(?:be)?\.(?:com|be)(?:\/watch\?v=|\/)([^\s&]+)/', $video_url, $matches ) ) {
		return $matches[1];
	}

	return false;
}

function vimeo_get_video_id( $video_url ) {
	if ( preg_match( '/(https?:\/\/)?(www\.)?(player\.)?vimeo\.com\/([a-z]*\/)*([0-9]{6,11})[?]?.*/', $video_url, $matches ) ) {
		return $matches[5];
	}

	return false;
}

function video_get_embed_code( $video_url, $user_options = null ) {
	static $player_id_increment = 0;
	$player_id_increment++;

	$service = video_get_service( $video_url );

	$default_options = array(
		'width' => 1280,
		'height' => null, // Calculates to proper aspect ratio, which would be 720.
		'autoplay' => false,
		'player_id' => 'video-player-' . $player_id_increment,

		// Vimeo Specific
	    'vimeo_autopause' => false,
	    'vimeo_badge' => false,
	    'vimeo_byline' => false,
	    'vimeo_color' => '00adef',
	    'vimeo_portrait' => false,
	    'vimeo_title' => false,

		// Youtube Specific
	    'youtube_autohide' => 2,
	    'youtube_color' => 'red',
	    'youtube_enablejsapi' => 1,
	    'youtube_modestbranding' => 1,
	);
	$options = wp_parse_args( $user_options, $default_options );

	if ( $options['height'] === null ) {
		$options['height'] = round( $options['width'] * (720/1280) );
	}

	if ( $service == 'vimeo' ) {

		$video_id = vimeo_get_video_id( $video_url );
		if ( !$video_id ) return false;

		$iframe_args = array(
			'autoplay'  => $options['autoplay'] ? '1' : '0',

			'autopause' => $options['vimeo_autopause'] ? '1' : '0',
			'badge'     => $options['vimeo_badge'] ? '1' : '0',
			'byline'    => $options['vimeo_byline'] ? '1' : '0',
			'color'     => substr(str_replace('#', '', $options['vimeo_color']), 0, 6),
			'player_id' => $options['player_id'],
			'portrait'  => $options['vimeo_portrait'] ? '1' : '0',
			'title'     => $options['vimeo_title'] ? '1' : '0',
		);

		$player_url = add_query_arg( $iframe_args, '//player.vimeo.com/video/' . $video_id );

		return sprintf(
			'<iframe id="%s" src="%s" width="%s" height="%s" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>',
			esc_attr( $options['player_id'] ),
			esc_attr( $player_url ),
			esc_attr( $options['width'] ),
			esc_attr( $options['height'] )
		);
	}

	if ( $service == 'youtube' ) {
		$video_id = youtube_get_video_id( $video_url );
		if ( !$video_id ) return false;

		$iframe_args = array(
			'autoplay' => $options['autoplay'] ? '1' : '0',
			'origin' => site_url(),

			'color' => $options['youtube_color'] == 'white' ? 'white' : 'red',
			'autohide' => (int) $options['youtube_autohide'],
			'enablejsapi' => $options['youtube_enablejsapi'] ? '1' : '0',
			'playerapiid' => $options['player_id'],
			'modestbranding' => $options['youtube_modestbranding'] ? '1' : '0',
		);

		$player_url = add_query_arg( $iframe_args, '//www.youtube.com/embed/' . $video_id );

		return sprintf(
			'<iframe id="%s" src="%s" width="%s" height="%s" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>',
			esc_attr( $options['player_id'] ),
			esc_attr( $player_url ),
			esc_attr( $options['width'] ),
			esc_attr( $options['height'] )
		);
	}

	return false;
}

function video_get_image( $video_url ) {
	$service = video_get_service( $video_url );

	if ( $service == 'vimeo' ) {
		$data = vimeo_get_image_attachment( $video_url );
	}else if ( $service == 'youtube' ) {
		$data = youtube_get_image_attachment( $video_url );
	}else{
		return false;
	}

	return empty($data['attachment_id']) ? false : $data['attachment_id'];
}

function youtube_get_image_attachment( $url ) {
	$video_id = youtube_get_video_id( $url );
	if ( !$video_id ) return false;

	$video_image_option = 'youtube-image-' . $video_id;

	// Return the image that has already been uploaded for this video ID
	if ( $v = get_option( $video_image_option ) ) {
		if ( isset($v['attachment_id']) && get_post( (int) $v['attachment_id'] ) ) {
			return $v;
		}
	}

	$image_url = 'http://img.youtube.com/vi/'. $video_id .'/0.jpg';

	$attachment = ld_handle_upload_from_url( $image_url );

	if ( !empty($attachment['attachment_id']) ) {
		update_option( $video_image_option, $attachment, false );
		return $attachment;
	}

	return false;
}

function vimeo_get_image_attachment( $url ) {
	$video_id = vimeo_get_video_id( $url );
	if ( !$video_id ) return false;

	$video_image_option = 'vimeo-image-' . $video_id;

	// Return the image that has already been uploaded for this video ID
	if ( $v = get_option( $video_image_option ) ) {
		if ( isset($v['attachment_id']) && get_post( (int) $v['attachment_id'] ) ) {
			return $v;
		}
	}

	$video_data = vimeo_get_info_by_url( $url );

	if ( $video_data && !empty($video_data['thumbnail_url']) ) {
		$image_url = $video_data['thumbnail_url'];

		$attachment = ld_handle_upload_from_url( $image_url );

		if ( !empty($attachment['attachment_id']) ) {
			update_option( $video_image_option, $attachment, false );
			return $attachment;
		}
	}

	return false;
}

function vimeo_get_info_by_url( $url, $option_id = null ) {
	/*
	Here is the result on a successful request, and also what will be cached. The ellipsis are added by me, you'll get full responses:
	{
	  type:              "video",
	  version:           "1.0",
	  provider_name:     "Vimeo",
	  provider_url:      "https://vimeo.com/",
	  title:             "PORTRAIT",
	  author_name:       "MILKYEYES - donato sansone",
	  author_url:        "http://vimeo.com/milkyeyes",
	  is_plus:           "0",
	  html:              "<iframe src="//player.vimeo.com/video/84241262" width="1280" ...></iframe>",
	  width:             1280,
	  height:            600,
	  duration:          171,
	  description:       "A slow and surreal video|slideshow of nightmareish, grotesque and a...",
	  thumbnail_url:     "http://b.vimeocdn.com/ts/461/023/461023899_1280.jpg",
	  thumbnail_width:   1280,
	  thumbnail_height:  600,
	  video_id:          84241262

	  // WordPress imports the thumbnail, giving you a full size embed code as well as the attachment ID so you can obtain your own.
	  // Note that this image is attached to the post obtained by get_the_ID()
	  media:             "<img src=\"http://dyscover.limelightmethod.com/wp-content/uploads/2013/10/461023899_128020.jpg\" alt=\"Vimeo: PORTRAIT\">",
	  media_id:          437,
	}
	*/

	// If no specific option ID is provided, create our own using the URL as input.
	if ( $option_id === null ) {
		$option_id = 'vimeo-' . sanitize_title( preg_replace( '/(https?:\/\/(www\.)?vimeo\.com\/)?/', '', $url ) );
	}
	if ( $option_id == 'vimeo-' ) {
		return false;
	}

	// Get the results from cache if we have it
	$from_cache = get_option( $option_id );
	if ( $from_cache ) {
		return $from_cache;
	}

	// We don't have cache, perform an oembed API request
	$oembed_url = sprintf( 'http://vimeo.com/api/oembed.json?url=%s', $url );

	$ch = curl_init();
	curl_setopt( $ch, CURLOPT_URL, $oembed_url );
	curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
	curl_setopt( $ch, CURLOPT_SSL_VERIFYHOST, false );
	curl_setopt( $ch, CURLOPT_SSL_VERIFYPEER, false );
	curl_setopt( $ch, CURLOPT_CONNECTTIMEOUT, 10 );
	$data_raw = curl_exec( $ch );
	curl_close( $ch );
	if ( !$data_raw ) {
		return false;
	}

	$data = json_decode( $data_raw, true );
	if ( !$data ) {
		return false;
	}

	if ( $data['thumbnail_url'] ) {
		// Sideload image using media_sideload_image: http://codex.wordpress.org/Function_Reference/media_sideload_image
		require_once( ABSPATH . 'wp-admin/includes/media.php' );
		require_once( ABSPATH . 'wp-admin/includes/file.php' );
		require_once( ABSPATH . 'wp-admin/includes/image.php' );

		$data['media'] = media_sideload_image( $data['thumbnail_url'], get_the_ID(), 'Vimeo: ' . $data['title'] );
		$data['media_id'] = image_get_attachment_id( $data['media'] );
	}

	update_option( $option_id, $data, false );

	return $data;
}









/**
 * Retrives an image from a URL and uploads it using ld_handle_upload_from_path. See that function for more details.
 *
 * Note: This function should also work for local file paths as well, but the implementation is slightly different than ld_handle_upload_from_path.
 *
 * @param $image_url
 * @param int $attach_to_post
 * @param bool|true $add_to_media
 * @return array
 */
function ld_handle_upload_from_url( $image_url, $attach_to_post = 0, $add_to_media = true ) {
	$remote_image = fopen($image_url, 'r');

	if ( !$remote_image ) return false;

	$meta = stream_get_meta_data( $remote_image );

	$image_meta = false;
	$image_filetype = false;

	if ( $meta && !empty($meta['wrapper_data']) ) {
		foreach( $meta['wrapper_data'] as $v ) {
			if ( preg_match('/Content\-Type: ?((image)\/?(jpe?g|png|gif|bmp))/i', $v, $matches ) ) {
			     $image_meta = $matches[1];
			     $image_filetype = $matches[3];
		     }
		}
	}

	// Resource did not provide an image.
	if ( !$image_meta ) return false;

	$v = basename($image_url);
	if ( $v && strlen($v) > 6 ) {
		// Create a filename from the URL's file, if it is long enough
		$path = $v;
	}else{
		// Short filenames should use the path from the URL (not domain)
		$url_parsed = parse_url( $image_url );
		$path = isset($url_parsed['path']) ? $url_parsed['path'] : $image_url;
	}

	$path = preg_replace('/(https?:|\/|www\.|\.[a-zA-Z]{2,4}$)/i', '', $path );
	$filename_no_ext = sanitize_title_with_dashes( $path, '', 'save' );

	$extension = $image_filetype;
	$filename = $filename_no_ext . "." . $extension;

	// Simulate uploading a file through $_FILES. We need a temporary file for this.
	$stream_content = stream_get_contents( $remote_image );

	$tmp = tmpfile();
	$tmp_path = stream_get_meta_data( $tmp )['uri'];
	fwrite( $tmp, $stream_content );
	fseek( $tmp, 0 ); // If we don't do this, WordPress thinks the file is empty

	$fake_FILE = array(
		'name'     => $filename,
		'type'     => 'image/' . $extension,
		'tmp_name' => $tmp_path,
		'error'    => UPLOAD_ERR_OK,
		'size'     => strlen( $stream_content ),
	);

	// Trick is_uploaded_file() by adding it to the superglobal
	$_FILES[basename( $tmp_path )] = $fake_FILE;

	// For wp_handle_upload to work:
	include_once ABSPATH . 'wp-admin/includes/media.php';
	include_once ABSPATH . 'wp-admin/includes/file.php';
	include_once ABSPATH . 'wp-admin/includes/image.php';

	$result = wp_handle_upload( $fake_FILE, array(
			'test_form' => false,
			'action'    => 'local',
	) );

	fclose( $tmp ); // Close tmp file
	@unlink( $tmp_path ); // Delete the tmp file. Closing it should also delete it, so hide any warnings with @
	unset( $_FILES[basename( $tmp_path )] ); // Clean up our $_FILES mess.

	fclose( $remote_image ); // Close the opened image resource

	$result['attachment_id'] = 0;

	if ( empty( $result['error'] ) && $add_to_media ) {
		$args = array(
				'post_title'     => $filename_no_ext,
				'post_content'   => '',
				'post_status'    => 'publish',
				'post_mime_type' => $result['type'],
		);

		$result['attachment_id'] = wp_insert_attachment( $args, $result['file'], $attach_to_post );

		$attach_data = wp_generate_attachment_metadata( $result['attachment_id'], $result['file'] );
		wp_update_attachment_metadata( $result['attachment_id'], $attach_data );

		if ( is_wp_error( $result['attachment_id'] ) ) {
			$result['attachment_id'] = 0;
		}
	}

	return $result;
}

/**
 * Takes a path to a file, simulates an upload and passes it through wp_handle_upload. If $add_to_media
 * is set to true (default), the file will appear under Media in the dashboard. Otherwise, it's hidden,
 * but stored in the uploads folder.
 *
 * Return Values: Similar to wp_handle_upload, but with attachment_id:
 *  - Success: Returns an array including file, url, type, attachment_id.
 *  - Failure: Returns an array with the key "error" and a value including the error message.
 *
 * @param $path
 * @param int $attach_to_post
 * @param bool $add_to_media
 *
 * @return array
 */
function ld_handle_upload_from_path( $path, $attach_to_post = 0, $add_to_media = true ) {
	if ( !file_exists( $path ) ) {
		return array( 'error' => 'File does not exist.' );
	}

	$filename = basename( $path );
	$filename_no_ext = pathinfo( $path, PATHINFO_FILENAME );
	$extension = pathinfo( $path, PATHINFO_EXTENSION );

	// Simulate uploading a file through $_FILES. We need a temporary file for this.
	$tmp = tmpfile();
	$tmp_path = stream_get_meta_data( $tmp )['uri'];
	fwrite( $tmp, file_get_contents( $path ) );
	fseek( $tmp, 0 ); // If we don't do this, WordPress thinks the file is empty

	$fake_FILE = array(
		'name'     => $filename,
		'type'     => 'image/' . $extension,
		'tmp_name' => $tmp_path,
		'error'    => UPLOAD_ERR_OK,
		'size'     => filesize( $path ),
	);

	// Trick is_uploaded_file() by adding it to the superglobal
	$_FILES[basename( $tmp_path )] = $fake_FILE;

	// For wp_handle_upload to work:
	include_once ABSPATH . 'wp-admin/includes/media.php';
	include_once ABSPATH . 'wp-admin/includes/file.php';
	include_once ABSPATH . 'wp-admin/includes/image.php';

	$result = wp_handle_upload( $fake_FILE, array(
		'test_form' => false,
		'action'    => 'local',
	) );

	fclose( $tmp ); // Close tmp file
	@unlink( $tmp_path ); // Delete the tmp file. Closing it should also delete it, so hide any warnings with @
	unset( $_FILES[basename( $tmp_path )] ); // Clean up our $_FILES mess.

	$result['attachment_id'] = 0;

	if ( empty( $result['error'] ) && $add_to_media ) {
		$args = array(
			'post_title'     => $filename_no_ext,
			'post_content'   => '',
			'post_status'    => 'publish',
			'post_mime_type' => $result['type'],
		);

		$result['attachment_id'] = wp_insert_attachment( $args, $result['file'], $attach_to_post );

		$attach_data = wp_generate_attachment_metadata( $result['attachment_id'], $result['file'] );
		wp_update_attachment_metadata( $result['attachment_id'], $attach_data );

		if ( is_wp_error( $result['attachment_id'] ) ) {
			$result['attachment_id'] = 0;
		}
	}

	return $result;
}

Yeah but take a look at oembed, a standardized way to get video thumbnails and other data:

https://wordpress.org/support/article/embeds/

I would not recommend using my original code, it's quite old

I ended up using oembed using ACF's video field, and pulling images from there, and adding as a featured image on save. Thanks for the answer!