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Modifying headers

From: PHP with MySQL Essential Training

Video: Modifying headers

In the movie following this one, I want us to talk about page redirection. But in order to do that, we first need to talk about headers because headers are the mechanisms that we'll use for page redirection. We're going to get into some technical details here. But bare with me because this is important to understand and there is a payoff at the end when we learn how to do redirection. To understand headers, let's go back to the diagram of the request response cycle that we looked at early on. When our web server responds to a browser request, we know that it sends back an HTML page. It also sends data just before the page that provides basic information to the browser about what to expect from the data that's going to follow.

Modifying headers

In the movie following this one, I want us to talk about page redirection. But in order to do that, we first need to talk about headers because headers are the mechanisms that we'll use for page redirection. We're going to get into some technical details here. But bare with me because this is important to understand and there is a payoff at the end when we learn how to do redirection. To understand headers, let's go back to the diagram of the request response cycle that we looked at early on. When our web server responds to a browser request, we know that it sends back an HTML page. It also sends data just before the page that provides basic information to the browser about what to expect from the data that's going to follow.

This data is called the HTTP header. And not only does it go from our web server to the browser with a response the browser actually send and http header with its request to the web server to begin with. Its because the header is part of the http protocol that all communication on the web uses. Here's a common example of an http header In the first line it tells us what version of http protocol it's using. And then it gives us a status code. This is very important. This number indicates whether or not this was a successful response or not.

200 means it was. That the page did successfully load. Other common status codes you would see here, would be 404 which is for Page Not Found. You're probably familiar with that. And then 500 would be for errors, things that have gone wrong. There at list a dozen other ones. But those are the three main ones that you would get there. And then you can see after that we have pairs of data. We have a label or an attribute followed by a colon, a space and then a value for it. So we have the date and we have the server, the content type, content length and then the fact letting us know the connection is no longer open.

Now usually you don't have to think about or even see this header information. It's automatically constructed for you by your web server when it responds to the request. But we can use php to give instructions to the web server to tell it how we want it to construct the header. We do that by using the header function. So header followed by a string. So for example, header, Content Type: Application PDF. Here we would be telling it, hey when you send this back, this is not HTML that you're sending, it's a PDF. You should treat it like a PDF when it arrives, and in fact if your browser has configuration options, it let's you decide how do you want to handle content that comes back as application PDF. You can also tell it that it's going to be an attachment and you can suggest a file name for what that attachment ought to be or we could send status codes like we talked about the 404.

And the 500 status codes that we could send back. Notice that those don't use that colon format, they just have HTTP and then a space followed by the status. Now here's the super duper important thing that you have to remember about headers. They come before all other page data. Remember they tell the browser here's what kind of data is about to come to you. And are required by the HTTP standard to proceed any communication back and forth. That means that any changes that we want to make to the header have to be done before we output anything from our file.

If we send even a single character to the user's browser, then the header is already on its way out the door right ahead of it. And at that point, it's too late to make changes to the header information, and I mean anything, even a blank space or a line return. Those count. And you also have to watch out for spaces and line returns in included files; those can cause you problems. It can come inside of a php block that includes white space. That doesn't matter. As long as the php doesn't output anything to the HTML. It's okay to turn on php tags, then have some white space. And then do your redirect.

That's not an issue. It's only HTML. Things that would be output to the browser, that would warrant a response. Back to the browser from the web server. At that point web server will already have started constructing its response back. Let's try an example. I'm going to open up basic.htm and we'll do save as. And let's just call this headers.php. Headers. And if we want to change the header All we have to do is say php, and then header http colon space 1.1 slash 404 not found semicolon.

Okay, but what's the problem with this? It's not the first thing. By the time that we get here the web server has already read all of this and already started processing it. Let's go ahead and save it and bring it up and try it. We'll go back here to Firefox and in the Sandbox we're going to load up headers. Warning can not modify header information. Headers already sent by and it tells us the output started here.

So, that's the warning that you're going to get, that's the common thing that you'll see, headers already sent, cannot modify that information. And that's why this has to be the very first thing. So let's move it up here at the top. Actually I'll leave this one here, just so you see, this won't work and then let's just paste it up here at the top. And now there's no space here at all, the first thing is the php tag. Spaces don't matter, it's only the fact that we don't have any HTML before it. Let's save it and let's go back and reload the page. It happily renders the page for us now, now if you didn't have that happen, if in fact it worked for you down here, it may be that you have output buffering turned on.

So we'll come back and talk about that. For now though let's assume that we all have it off and let's put it up here at the top. Now, how do we see this? We sent it, it came back but we don't have a way in our browser really to see it. You can use Firebug. It's a nice tool that goes with Firefox. You can see it that way. But PHP also lets us see it. I'm going to use pretags and I'm going to call a function here. We don't really need to remember this function because you will never ever need it again. But it is useful just in this one context and that is headers_list. And that will return a list of the headers to us. And I'll use print r with that.

Oops put parenthesis around it. There we go. And that is going to return, then, an array, that shows us what we're getting back as the headers. Now, the headers here did not include the status, that status wasn't one of those, but we can change something else about it. Let's say header, it reported back my version of PHP. That's something that's important to note, that it does tell other web server what versions of PHP you're running. A common thing that people do is change it, so that it doesn't. There we go. And we could say, x powered by none of your business.

So there we go. Let's come back and let's reload the page. X powered by none of your business. So that's it. That's all there is to being able to set headers. The most important thing is just remembering that no space can come before those headers, unless you have output buffering turned on. Now, we're not going to be setting headers very often. It's really a rare kind of thing that you'll do, in this context. But there's one time that we do do it a lot and that's for redirects, and that's what we'll talk about in the next movie.

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This video is part of

Image for PHP with MySQL Essential Training
PHP with MySQL Essential Training

131 video lessons · 34457 viewers

Kevin Skoglund
Author

 
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  1. 4m 8s
    1. Welcome
      1m 0s
    2. Using the exercise files
      3m 8s
  2. 15m 6s
    1. What is PHP?
      3m 52s
    2. The history of PHP
      2m 51s
    3. Why choose PHP?
      4m 10s
    4. Installation overview
      4m 13s
  3. 54m 53s
    1. Overview
      2m 33s
    2. Working with Apache Web Server
      6m 56s
    3. Changing the document root
      7m 24s
    4. Enabling PHP
      6m 16s
    5. Upgrading PHP
      3m 30s
    6. Configuring PHP
      10m 3s
    7. Installing MySQL
      5m 46s
    8. Configuring MySQL
      7m 24s
    9. Text editor
      5m 1s
  4. 31m 25s
    1. Overview
      3m 27s
    2. Installing WampServer
      5m 46s
    3. Finding the document root
      2m 24s
    4. Configuring PHP
      8m 12s
    5. Configuring MySQL
      5m 45s
    6. Text editor
      5m 51s
  5. 19m 12s
    1. Embedding PHP code on a page
      6m 43s
    2. Outputting dynamic text
      5m 55s
    3. The operational trail
      2m 27s
    4. Inserting code comments
      4m 7s
  6. 1h 18m
    1. Variables
      7m 50s
    2. Strings
      4m 38s
    3. String functions
      8m 54s
    4. Numbers part one: Integers
      6m 27s
    5. Numbers part two: Floating points
      5m 25s
    6. Arrays
      10m 0s
    7. Associative arrays
      6m 37s
    8. Array functions
      6m 33s
    9. Booleans
      3m 50s
    10. NULL and empty
      5m 15s
    11. Type juggling and casting
      8m 27s
    12. Constants
      4m 43s
  7. 27m 37s
    1. If statements
      6m 0s
    2. Else and elseif statements
      4m 16s
    3. Logical operators
      7m 30s
    4. Switch statements
      9m 51s
  8. 42m 15s
    1. While loops
      8m 41s
    2. For loops
      5m 59s
    3. Foreach loops
      8m 16s
    4. Continue
      8m 28s
    5. Break
      4m 8s
    6. Understanding array pointers
      6m 43s
  9. 37m 25s
    1. Defining functions
      8m 25s
    2. Function arguments
      5m 32s
    3. Returning values from a function
      7m 33s
    4. Multiple return values
      4m 53s
    5. Scope and global variables
      6m 2s
    6. Setting default argument values
      5m 0s
  10. 20m 18s
    1. Common problems
      3m 47s
    2. Warnings and errors
      8m 36s
    3. Debugging and troubleshooting
      7m 55s
  11. 57m 57s
    1. Links and URLs
      5m 33s
    2. Using GET values
      5m 35s
    3. Encoding GET values
      8m 41s
    4. Encoding for HTML
      9m 26s
    5. Including and requiring files
      7m 40s
    6. Modifying headers
      6m 45s
    7. Page redirection
      6m 43s
    8. Output buffering
      7m 34s
  12. 1h 3m
    1. Building forms
      7m 28s
    2. Detecting form submissions
      5m 59s
    3. Single-page form processing
      7m 57s
    4. Validating form values
      10m 40s
    5. Problems with validation logic
      9m 54s
    6. Displaying validation errors
      7m 23s
    7. Custom validation functions
      6m 28s
    8. Single-page form with validations
      7m 25s
  13. 28m 5s
    1. Working with cookies
      2m 49s
    2. Setting cookie values
      5m 55s
    3. Reading cookie values
      6m 1s
    4. Unsetting cookie values
      4m 51s
    5. Working with sessions
      8m 29s
  14. 48m 39s
    1. MySQL introduction
      6m 43s
    2. Creating a database
      7m 41s
    3. Creating a database table
      7m 42s
    4. CRUD in MySQL
      5m 48s
    5. Populating a MySQL database
      7m 32s
    6. Relational database tables
      6m 40s
    7. Populating the relational table
      6m 33s
  15. 56m 4s
    1. Database APIs in PHP
      4m 51s
    2. Connecting to MySQL with PHP
      7m 45s
    3. Retrieving data from MySQL
      8m 47s
    4. Working with retrieved data
      6m 12s
    5. Creating records with PHP
      6m 58s
    6. Updating and deleting records with PHP
      9m 6s
    7. SQL injection
      3m 5s
    8. Escaping strings for MySQL
      6m 45s
    9. Introducing prepared statements
      2m 35s
  16. 35m 58s
    1. Blueprinting the application
      7m 19s
    2. Building the CMS database
      5m 14s
    3. Establishing your work area
      4m 38s
    4. Creating and styling the first page
      4m 22s
    5. Making page assets reusable
      6m 36s
    6. Connecting the application to the database
      7m 49s
  17. 32m 49s
    1. Adding pages to the navigation subjects
      5m 58s
    2. Refactoring the navigation
      6m 7s
    3. Selecting pages from the navigation
      6m 2s
    4. Highlighting the current page
      5m 26s
    5. Moving the navigation to a function
      9m 16s
  18. 1h 45m
    1. Finding a subject in the database
      9m 48s
    2. Refactoring the page selection
      10m 52s
    3. Creating a new subject form
      6m 55s
    4. Processing form values and adding subjects
      11m 20s
    5. Passing data in the session
      9m 16s
    6. Validating form values
      9m 40s
    7. Creating an edit subject form
      8m 30s
    8. Using single-page submission
      7m 44s
    9. Deleting a subject
      9m 44s
    10. Cleaning up
      10m 37s
    11. Assignment: Pages CRUD
      4m 30s
    12. Assignment results: Pages CRUD
      6m 10s
  19. 39m 26s
    1. The public appearance
      8m 52s
    2. Using a context for conditional code
      11m 37s
    3. Adding a default subject behavior
      6m 9s
    4. The public content area
      5m 51s
    5. Protecting page visibility
      6m 57s
  20. 1h 3m
    1. User authentication overview
      4m 3s
    2. Admin CRUD
      8m 41s
    3. Encrypting passwords
      7m 26s
    4. Salting passwords
      5m 42s
    5. Adding password encryption to CMS
      11m 54s
    6. New PHP password functions
      3m 13s
    7. Creating a login system
      11m 28s
    8. Checking for authorization
      5m 48s
    9. Creating a logout page
      5m 40s
  21. 2m 4s
    1. Next steps
      2m 4s

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