PHP Interview Questions And Answers

Understanding resources

If you are confused by what exactly is meant by the term “resource” in the question above, then you should just think of a web resource as a generic term for a file. So, a CSS file, a Javascript file, an HTML file, a SWF (a file used for Adobe Flash) file, an image file (jpg, png, etc) – each of these is a different type of resource, and as you know there are many more types of resources on the web.

The difference between single file resources and other resources

The question specifically calls HTML files complex resources because of the simple fact that HTML documents are complex – they can contain many references to single file resources like image files, and SWF files. A single file resource does not contain references to other resources – a jpg or gif file can not contain a reference to another file, and that is why they are both considered single file resources. An HTML file, on the other hand, is also considered a resource itself, but because it contains references to other resources, it is not considered to be a single file resource.
In order to retrieve a resource from the web server where that resource is stored, a web browser has to make an HTTP request. Read on to understand more about HTTP requests.

What exactly is an HTTP request?

The question asks for two major things from a URL – the total number of HTTP requests and the total download size for all requests. The download size is easy enough to understand, but you may be confused by what exactly is meant by an HTTP request. HTTP is the protocol used to communicate on the web. When you visit a webpage, your browser will make an HTTP request to the server that hosts that webpage, and the server on which the webpage is hosted will respond with an HTTP response.
But, what is important to understand here, is that your browser will probably have to make multiple HTTP requests in order to retrieve a single HTML page at a given URL, because that webpage will probably have some CSS files to go along with it, some Javascript files, and probably some images as well. Each one of those resources is a separate HTTP request – 2 image files, 2 Javascript files, and 2 CSS files means 6 separate HTTP requests. In HTTP, only one resource can be requested at a time – so we can not have 1 request for 6 different resources, instead we must have 6 requests for those 6 different resources.
So, for the purpose of this interview question, we have to find out the number of HTTP requests that will be made for a given URL – hopefully what that means is now clear to you. We’ll go more in depth on this later – and show some actual code – as we cover some other things as well.

How to find the download size of a file?

The question also asks us to find the total download size of a URL. But what if that URL passed into the script just points to a single file resource like a JPG file or a GIF file? Well, for a single file resource we just need to find the size of that particular file and then return it as the answer, and we are done. But, for an HTML document we will need to find the total size of all resources that are embedded and included on the page and return that as the answer – because you must remember that we want the total download size of a URL.
So, let’s write a PHP function that will return the download size of a single file resource. How should we approach writing this function – what is the easiest way to find the download size of a single file resource on the web?
Well, there is an HTTP header called “Content-Length” which will actually tell us the size of a particular resource file in the HTTP response (after the resource is requested). So, all we have to do is use PHP’s built in “get_headers” function, which will retrieve all the HTTP headers sent by the server in response to an HTTP request.
The get_headers function accepts a URL as an argument. So, the PHP code to retrieve the “Content-Length” header would look like this:

function get_remote_file_size($url) {

$headers = get_headers($url, 1);
    
if (isset($headers['Content-Length'])) 
       return $headers['Content-Length'];
    
    //checks for lower case "L" in Content-length:
if (isset($headers['Content-length'])) 
       return $headers['Content-length'];


}

But, there is actually a problem with this code: you will not always receive the Content-Length header in an HTTP response. In other words, the HTTP Content-Length header is not guaranteed to be sent back by the web server hosting that particular URL, because it depends on the configuration of the server. This means that you need an alternative that always works in case the approach above fails.

An alternative to using the content-length header

Well, we can actually download the file ourselves and then just get the download size for that URL. How can we do this? Well, this is where we can use cURL as we discussed above. Once we download the resource, we can retrieve the download size using the CURLINFO_SIZE_DOWNLOAD parameter. So, using this approach as a backup to our first approach, we can come up with this code (the code in red below is the new code):

function get_remote_file_size($url) {

 $headers = get_headers($url, 1);
    
    if (isset($headers['Content-Length'])) 
       return $headers['Content-Length'];
    
    //checks for lower case "L" in Content-length:
    if (isset($headers['Content-length'])) 
       return $headers['Content-length'];

//the code below runs if no "Content-Length" header is found:


    $c = curl_init();
    curl_setopt_array($c, array(
        CURLOPT_URL => $url,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_HTTPHEADER => array('User-Agent: Mozilla/5.0 
        (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1.3) 
        Gecko/20090824 Firefox/3.5.3'),
        ));
    curl_exec($c);
    
    $size = curl_getinfo($c, CURLINFO_SIZE_DOWNLOAD);
    
    return $size;
        
    curl_close($c);

}

How should we parse HTML in PHP?

What exactly is meant by the sentence “For a complex resource such as an HTML document, the script would need to parse it to find references to embedded, included resources: javascript files, CSS files, iframes, etc.”?
Well, as you probably know, an HTML page often uses other files to render the HTML page – like CSS file(s) for styling, Javascript file(s) for adding more functionality to the HTML page, and so on. But the question is how do we take an HTML page and find all of those resources in the HTML page. Of course, this is easy to do if we are reading the HTML page with the human eye. But, we want to find these resources using a program that will read the HTML for us. This is actually more complicated than it seems – and the process by which a program (like PHP) reads an HTML file and analyzes the text to extract meaningful data (like resources) is known as parsing the HTML. Any text can be parsed, but we are exclusively focused on HTML for the purpose of this interview question.
Parsing HTML in PHP is definitely something that you do not want to do on your own, because it is so complex. The best way to parse HTML in PHP is to use a library that already exists – because writing an entire library from scratch to do this would obviously be considered way too much work for an answer to an interview question.
Note that the question states that “You may not use any 3rd-party code that performs the entire task described below”. This just means you can not use 3rd party code to perform the entire task – but using a PHP library to help you with part of this question is perfectly OK. Of course, you should clarify this with your interviewer if you are in doubt, but we know for sure that for this particular question there’s no way that the interviewers would be expecting you to perform this task without using a library to help you parse the HTML.
Note that the instructions say: “For a single-file resource such as an image or SWF, the script would simply report on the total size of the document.”
This means that if the URL is single file resources like an image file, we can just return the size of the file and we are done. But, how can we distinguish between a single-file resource and a non-single file resource? Well, we could just say that all non-HTML pages are single file resources. That statement is not entirely true, as you can read about in part 3, but we will pretend it is for the sake of keeping things simple.
But wait, you might be thinking – what about PHP, JSP, ASP and all of those pages? Well, of course there is some application specific logic embedded in those pages, but once those pages are rendered in a browser they are all HTML pages, regardless of what their file extension may be.
So, all we have to do in order to determine if a URL points to a single file resource is to see if it is an HTML page – if it is not an HTML page, then we know that the file is a single resource file.