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

When I was writing about using Thunder to download yesterday, 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 None/*

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

/**
 * 相对路径转网络绝对路径
 * @param string $file
 * @return string
 */
function dirToHttpUrl($file) {

	//判断文件是否存在
	if (!file_exists($file)) {
		return false;
	}

	//域名
	$nowUrl = dirname('http://'.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF']);	//当前域名
	$tempUrl = explode('.', $_SERVER['HTTP_HOST']);
	$dirUrl = 'http://www.'.$tempUrl[1].'.'.$tempUrl[2].'/';					//主域名

	//文件路径的层次统计
	$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);
	}

	//域名层次统计
	$tempWeb = explode('/', $nowUrl);
	$tempWeb = array_slice($tempWeb, 3);
	$webNum = count($tempWeb);

	//文件对应的域名
	if ($fileNum > $webNum) {
		$nowUrl = $dirUrl;
	}

	//返回
	return $nowUrl.$fileEnd;

}

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