Возможно ли скачивать YouTube ссылки через PHP с возможностью восстановления?

Попробуйте это, чтобы получить размер файла:

<?php
/**
* Returns the size of a file without downloading it, or -1 if the file
* size could not be determined.
*
* @param $url - The location of the remote file to download. Cannot
* be null or empty.
*
* @return The size of the file referenced by $url, or -1 if the size
* could not be determined.
*/
function curl_get_file_size( $url ) {
// Assume failure.
$result = -1;

$curl = curl_init( $url );

// Issue a HEAD request and follow any redirects.
curl_setopt( $curl, CURLOPT_NOBODY, true );
curl_setopt( $curl, CURLOPT_HEADER, true );
curl_setopt( $curl, CURLOPT_RETURNTRANSFER, true );
curl_setopt( $curl, CURLOPT_FOLLOWLOCATION, true );
curl_setopt( $curl, CURLOPT_USERAGENT, 'user_agent_string' );

$data = curl_exec( $curl );
curl_close( $curl );

if( $data ) {
$content_length = "unknown";
$status = "unknown";

if( preg_match( "/^HTTP\/1\.[01] (\d\d\d)/", $data, $matches ) ) {
$status = (int)$matches[1];
}

if( preg_match( "/Content-Length: (\d+)/", $data, $matches ) ) {
$content_length = (int)$matches[1];
}

// http://en.wikipedia.org/wiki/List_of_HTTP_status_codes
if( $status == 200 || ($status > 300 && $status <= 308) ) {
$result = $content_length;
}
}

return $result;
}

Использование:

$file_size = curl_get_file_size($url);