Anti-spam image
creation
Before you start this lesson, before read the lesson PHP
Image processing introduction and make sure you enable GD library.
This tutorial will demonstrate how to create Anti-Spam
images. This is an image contains (some random) text which is very,
very hard for any OCR program to decipher but easy for a human to
read. This is used to prevent automatic signup to mailing lists and
various on-line services.
Many users have run across the verification image containing
numbers and letters that you must plug into an input box for verification.
And maybe you thought, "How could I do that ?"
This tutorial will show you how.
Here is the anti-spam
image (image verification) live demo.
Create Image
Before we start this lesson, it is better to see
the whole flow chart of this verification procedure. First
we need create an image with a verification string in it and
meanwhile use a session variable to store this random generated
verification string. Then we compare user input with the session
variable. If match, pass. Other wise, prompt error message.
Now, it is our first step. We need create a verification
image, which is generated randomly on fly.
In this tutorial, we create an image with black
background and white foreground.
<?php
/* initialize a session. */
session_start();
/*We'll set this variable later.*/
$new_string;
/*random string generator.*/
/*The seed for the random number*/
//srand((double)microtime()*1000000);
/*Runs the string through the md5 function*/
$string = md5(rand(0,9999));
/*creates the new string. */
$new_string = substr($string, 17, 6);
//register session variable
$_SESSION['new_string']=$new_string;
/*You will need these two lines below.*/
/* set up image, the first number is the width and the second
is the height*/
$im = ImageCreate(200, 40);
/*creates two variables to store color*/
$white = ImageColorAllocate($im, 255, 255, 255);
$black = ImageColorAllocate($im, 0, 0, 0);
/*fill image with black*/
ImageFill($im, 0, 0, $black);
/*writes string */
ImageString($im, 4, 96, 19, $new_string, $white);
/* output to browser*/
Header("Content-Type: image/png");
ImagePNG($im);
ImageDestroy($im);
?> |
First step, we will create a random string generator to place
as the verification code inside the image. I won't discuss
how the random generator code works, but will only lay out
the code with comments.
Use $new_string storing 6 letters from
random generated string as verification string and meanwhile
save the verification string to session variable $_SESSION['new_string'].
Second step, we need write the verification string to image.
Use ImageCreate to create an image with size of 200 X 40 pixels.
ImageColorAllocate function is called twice. The first time
is to set the color for white, the second to set the color
for black. The function take 4 arguments: The image pointer
variable $im, and the RGB (red, green, blue) components that
are separated by commas. (255, 255, 255, is the code for white,
while 0, 0, 0, is the code for black. You can play with the
numbers to produce any color you want.)
Then, ImageFill function is called. It takes 4 arguments: Again we add the image
pointer variable $im, the second and third are x, y coordinates in our image,
0, 0, being at the top left corner. Our image size is 200x40, therefore, the
bottom right corner would be 200, 40. The fourth argument tell us with what color
to fill the image with: In our case it is black.
ImageString adds a string to the image. It takes
six arguments. The first is the image pointer variable, the
second argument can be any number from 1 to 5,(which calls
for one of the built in fonts). The next two arguments are
the coordinates, first the x (width) then the y (height). This
is for the placement of our string. The next argument calls
for the string variable. In our case it is $new_string. The
last argument is the variable containing the color, which is
$white.
The last step is to output the created image to browser. Header("Content-Type:
image/png"); tells the browser the image is in png format. Finally
ImageDestroy function is called , it has one argument: the pointer variable.
Destroying the image frees up server memory. Your server administrator will like
you for this.
Save the above code as createimage.php.
Input form
The following is the verification form to get the
user input.
<html>
<body>
<img src="createimage.php">
<br><br>
Type the code you see in the image in the box below. (case
sensitive)
<form action="formhandler.php" method=post>
<input name="random" type="text" value="">
<input type="submit">
</form>
</body>
</html>
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Verification code
Save the following code as formhandler.php.
If you have read the lesson: PHP
working with form, you may feel easy to see the following
code.
$random retrieves the user input $_POST['random']. Then compare
$random with the session variable $new_string
<?php
/*This starts the session that we created on the last page*/
session_start();
$random=$_POST['random'];
/*This trims the random variable of any white space that
the user may have unknowingly put in.*/
$random = trim($random);
/*
if the string that the user put into the text box is
equal to what is in the image then print to the screen, "You
are verified." If they are not equal it tells the
user to go back and try again.*/
/*We can use the variable $new_string because we registered
it into our session on the last page, it retains its
value that it had on the first page.*/
if ($_SESSION['new_string'] == $random){
echo "You are verified";
}
else
{
echo "Please go back and get verified.";
}
?>
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