Relative path to network absolute path (supports second-level domain name) ./a.jpg => http://www.w.com/a.jpg

Yesterday, when I was writing about using Thunder to download, I found that the ./a.jpg file converted to Thunder was not the network address, so I wrote a method to convert it. I am currently at the basic level of PHP, so I would like to ask for help if I have any deficiencies.
It will automatically determine the current domain name and main domain name. The following is the form:
./a.jpg => http://about.w.com/a.jpg;
../a.jpg => http:// www.w.com/a.jpg

/**
 * Convert relative path to network absolute path
 * @param string $file
 * @return string
*/
function dirToHttpUrl($file) {

 //Determine whether the file exists
 if (!file_exists($file)) {
 return false;
 }

 //Domain name
 $nowUrl = dirname('http://'.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF']); //Current domain name
 $tempUrl = explode('.', $_SERVER['HTTP_HOST' ]);
 $dirUrl = 'http://www.'.$tempUrl[1].'.'.$tempUrl[2].'/'; //Main domain name

 //Level statistics of file paths
 $tempFile = explode('../', $file);
 $tempNum = array_count_values($tempFile);

 if (array_key_exists('', $tempNum)) {
 $fileNum = $tempNum[''];
 $fileEnd = end($tempFile);
 } else {
 $fileNum = 0;
 $fileEnd = '/'.substr($tempFile[0], 2);
 }

 //Domain name level statistics
 $ tempWeb = explode('/', $nowUrl);
 $tempWeb = array_slice($tempWeb, 3);
 $webNum = count($tempWeb);

 //Domain name corresponding to the file
 if ($fileNum > $ webNum) {
 $nowUrl = $dirUrl;
 }

 //Return
 return $nowUrl.$fileEnd;

}

//dirToHttpUrl('./1.jpg');