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# PHP Web Concepts

This session demonstrates how PHP can provide dynamic content according to browser type, randomly generated numbers or User Input. It also demonstrated how the client borwser can be redirected.

## Identifying Browser & Platform

PHP creates some useful environment variables that can be seen in the phpinfo.php page that was used to setup the PHP environment.

One of the environemnt variables set by PHP is HTTP_USER_AGENT which identifies the user's browser and operating system.

PHP provides a function getenv() to access the value of all the environment variables. The information contained in the HTTP_USER_AGENT environment variable can be used to create dynamic content appropriate to the borwser.

Following example demonstrates how you can identify a client borwser and operating system.

NOTE: The function preg_match()is discussed in PHP Regular expression session.

    <html>
    <body>
    <?php
       $viewer = getenv( "HTTP_USER_AGENT" );
       $browser = "An unidentified browser";
       if( preg_match( "/MSIE/i", "$viewer" ) )
       {
          $browser = "Internet Explorer";
       }
       else if(  preg_match( "/Netscape/i", "$viewer" ) )
       {
          $browser = "Netscape";
       }
       else if(  preg_match( "/Mozilla/i", "$viewer" ) )
       {
          $browser = "Mozilla";
       }
       $platform = "An unidentified OS!";
       if( preg_match( "/Windows/i", "$viewer" ) )
       {
          $platform = "Windows!";
       }
       else if ( preg_match( "/Linux/i", "$viewer" ) )
       {
          $platform = "Linux!";
       }
       echo("You are using $browser on $platform");
    ?>
    </body>
    </html>

This is producing following result on my machine. This result may be different for your computer depnding on what you are using.

    You are using Mozilla! on Windows!

## Display Images Randomly

The PHP rand() function is used to generate a random number.i This function can generate numbers with-in a given range. The random number generator should be seeded to prevent a regular pattern of numbers being generated. This is achieved using the srand() function that specifiies the seed number as its argument.

Following example demonstrates how you can display different image each time out of four images:

    <html>
    <body>
    <?php
      srand( microtime() * 1000000 );
      $num = rand( 1, 4 );

      switch( $num )
      {
      case 1: $image_file = "/home/images/alfa.jpg";
              break;
      case 2: $image_file = "/home/images/ferrari.jpg";
              break;
      case 3: $image_file = "/home/images/jaguar.jpg";
              break;
      case 4: $image_file = "/home/images/porsche.jpg";
              break;
      }
      echo "Random Image : <img src=$image_file />";
    ?>
    </body>
    </html>

## Using HTML Forms

The most important thing to notice when dealing with HTML forms and PHP is that any form element in an HTML page will automatically be available to your PHP scripts.

Try out following example by putting the source code in test.php script.

    <?php
      if( $_POST["name"] || $_POST["age"] )
      {
         echo "Welcome ". $_POST['name']. "<br />";
         echo "You are ". $_POST['age']. " years old.";
         exit();
      }
    ?>
    <html>
    <body>
      <form action="<?php $_PHP_SELF ?>" method="POST">
      Name: <input type="text" name="name" />
      Age: <input type="text" name="age" />
      <input type="submit" />
      </form>
    </body>
    </html>

- The PHP default variable $_PHP_SELF is used for the PHP script name and when you click "submit" button then same PHP script will be called and will produce following result:

- The method = "POST" is used to post user data to the server script. There are two methods of posting data to the server script which are discussed in PHP GET & POST chapter.

## Browser Redirection

The PHP header() function supplies raw HTTP headers to the browser and can be used to redirect it to another location. The redirection script should be at the very top of the page to prevent any other part of the page from loading.

The target is specified by the Location: header as the argument to the header() function. After calling this function the exit() function can be used to halt parsing of rest of the code.

Following example demonstrates how you can redirect a borwser request to another web page. Try out this example by puttingthe source code in test.php script.

    <?php
      if( $_POST["location"] )
      {
         $location = $_POST["location"];
         header( "Location:$location" );
         exit();
      }
    ?>
    <html>
    <body>
       <p>Choose a site to visit :</p>
       <form action="<?php $_PHP_SELF ?>" method="POST">
       <select name="location">
          <option value="http://w3c.org">
                World Wise Web Consortium
          </option>
          <option value="http://www.google.com">
                Google Search Page
          </option>
       </select>
       <input type="submit" />
       </form>
    </body>
    </html>

## Displaying "File Download" Dialog Box

Sometime it is desired that you want to give option where a use will click a link and it will pop up a "File Download" box to the user in stead of displaying actual content. This is very easy and will be achived through HTTP header.

The HTTP header will be different from the actual header where we send Content-Type as text/html\n\n. In this case content type will be application/octet-stream and actual file name will be concatenated alongwith it.

For example,if you want make a FileName file downloadable from a given link then its syntax will be as follows.

    #!/usr/bin/perl

    # HTTP Header
    print "Content-Type:application/octet-stream; name=\"FileName\"\r\n";
    print "Content-Disposition: attachment; filename=\"FileName\"\r\n\n";

    # Actual File Content
    open( FILE, "<FileName" );
    while(read(FILE, $buffer, 100) )
    {
       print("$buffer");
    }

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