PHP

For most functions it is normal for have the function return the output of a calculation. With PHP it is also possible to pass values to the function by reference. A better way of saying this is rather than pass the value of the variable you pass a pointer to the variable itself. When you do this anything that you do to the variable inside the function is also done outside, so if you interact with the variable again it will contain a different value.

The ampersand (&) character is used in the parameters of the function to stipulate that a parameter will be passed by reference.

Here is an example of this at work.

Use the following function to list the contents of one or more nested directories.

function recursive_directory($dirname,$maxdepth=10, $depth=0){
 if ($depth >= $maxdepth) {
  return false;
 }
 $subdirectories = array();
 $files = array();
 if (is_dir($dirname) && is_readable($dirname)) {
  $d = dir($dirname);
  while (false !== ($f = $d->read())) {
   $file = $d->path.'/'.$f;
   // skip . and ..
   if (('.'==$f) || ('..'==$f)) {
    continue;
   };
   if (is_dir($dirname.'/'.$f)) {
    array_push($subdirectories,$dirname.'/'.$f);
   } else {
    array_push($files,$dirname.'/'.$f);
   };
  };
  $d->close();
  foreach ($subdirectories as $subdirectory) {
    $files = array_merge($files, recursive_directory($subdirectory, $maxdepth, $depth+1));
  };
 }
 return $files;
}

Use this in the following way.

Try running the following PHP script.

<?php
 echo 'browser output';
 session_start();
?>

You will either see normal output or get the following error messages.

This is because when you try to start the session it adds items to the headers outputted by the browser, including the setting up of cookies. To stop this happening you need to ensure that the session_start() function call is put before any output from the browser. This is the case for all header modifying functions including set_cookie() and header().

The set_error_handler() function can be used in PHP to allow you to catch any run time errors and act accordingly. This function can take two parameters:

• Error Handler : This a string which is the name of the function that will be called if an error is encountered.
• Error Types (optional) : This is an optional parameter used to tell PHP on what error codes to act. This is the same error reporting setting.

The function that is defined in the function must have the following footprint as a minimum.

function handler($errno,$errstr)

You can also get a lot more information out by using other parameters.

function handler($errno,$errstr,$errfile,$errline,$errcontext)

Here is some code that will print two errors to screen with as much information as possible.

Appending arrays in PHP can be done with the array_merge() function. The array will take any number of arrays as arguments and will return a single array. Here is an example using just two arrays.

$array1 = array('item1', 'item2');
$array2 = array('item3', 'item4');
$array3 = array_merge($array1, $array2);
print_r($array3);

Will print out.

Array
(
 [0] => item1
 [1] => item2
 [2] => item3
 [3] => item4
)

You can also create arrays using the array command inside the parameter list.

The following code loads the contents of a text file and randomly displays a line from it. You can use this to display a random quote on a page every time it loads.

$file= "quotes.txt";
$quotes = file($file);
srand((double)microtime()*1000000);
$randomquote = rand(0, count($quotes)-1);
echo $quotes[$randomquote];

It works by loading a file into memory and picking a line from that file at random. Here is a sample file you might want to use.

This is the first quote - Person One
This is the second quote - Person Two

Fill this with your own quotes and you are away.

The singleton design pattern is used to centralise an object in an application that is used to store changing variables that can then be accessed by other parts of the program. It allows only the single instantiation of an object, hence the name.

The main use of a singleton is to create an alternative to the use of global variables. Although global variables are useful they can lead to a major problem if you happen to assign two variables with the same name, PHP generates no errors and your program will start to act oddly. This might not be a problem in small programs, but in larger systems it is very easy to have global variable clashes.

The singleton pattern gets around this by using an single object that is accessible to any part of the program that wants it. If two classes are declared with the same name PHP throws an error so if another part of the program tries to use the same class name you will know about it.

Every time you accept any input from a use you should attempt to validate it. This is to stop users trying to break the site and also corrects silly mistakes that users might introduce to their input.

Before sending off an email to a new user congratulating them on signing up it is best to validate that email address. Here is function that does this.

function validateEmail($email)
{
  $reg_exp = '/^[A-z0-9][\w.-]*@[A-z0-9][\w\-\.]+\.[A-z0-9]{2,3}$/';
  if (preg_match($reg_exp, $email) == true) {
    return true;
  } else {
    return false;
  }
}

This can be used in the following way.

The Exif/IFD0 functions in PHP work with images to pull out meta data associated with them. Most image applications and digital cameras will produce an image with a certain amount of meta data present. This is obvious stuff like file size and creation time stamps, but you can also get stuff like copy right notices, camera name, date picture taken and even things like location if the camera was linked to a GPS system. This meta data can be used to sort or categorise images.

Installation

On Linux systems you must configure exif support with the command --enable-exif when calling the configure script.

On Windows all you have to do is uncomment the lines in your php.ini file for the DLL's php_exif.dll and php_mbstring.dll. However, you must ensure that the php_mbstring.dll DLL is loaded before the php_exif.dll DLL. So you will need to edit your php.ini file so that php_mbstring.dll is located above php_exif.dll.

To detect a user returning to a web page you can use the built in PHP session manager. At the start of the code you can use the session_start() function to initiate the session, you can then use the $_SESSION global array to store and retrieve information. The session_start() function sends a cookie to the client with a unique code that looks like this

8a9af5644326881594811db6fe96faf8

The session variable information is kept in a file on the web server and when the session_start() function is called PHP looks for the file with the corresponding name. It then parses this file and loads the variables into memory. This is all done behind the scenes by PHP, all you need to know is that you can set values in the $_SESSION array and get them back the next time the page is loaded.