Creating a Basic Captcha script

Nowadays sites offer Captcha systems where you just have to include a javascript file, but ill teach you how to make a basic captcha.

This is what we are making:

Refresh and watch the captcha change.

In PHP make sure you have these brackets between your code
[PHP]<?php
/r code here
?>
[/PHP]
First step is to create our variables. So lets call our Letter length, and then give the characters to use as letters (the alphabets).

[PHP]
$length = 3;
$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVW XYZ';
[/PHP]
Change the Length to how many letters you want to show. I chose 3.

Now we need to count the amount of chars we have and add an empty var for now. For that we will use
strlen()
[PHP
$charslength = strlen($chars);
$randomstring = '';
[/PHP]
Here $charslength is the Chars length, which in this case is 52.

Next is a loop, ill try to explain the loop to you.
[PHP]
for ($i = 0; $i < $length; $i++) $randomstring .= substr($chars, rand(0, $charslength - 1), 1);[/PHP]
The loop is telling us that substr() is getting a character from the $char variable using a random number between 0 and Chars Length, which is 52.

Now by now $randomstring will now equal 3 random letters, but we aren't dont yet. Next step is mixing the letters with 3 digits.

[PHP]$number = rand(100, 999);
$captcha = $number . $randomstring;
$captcha = str_shuffle($captcha);
[/PHP]
This is telling us that $number will be a random number between 100-999 (3 digits), if you want more add a 0 to the left and a 9 to the right one.
Next we have $captcha, Captcha equals the numbers with the letters.
So Captcha would look something like 209FoW.
So what we are doing now we have $Captcha equal the previous $captcha but shuffled, so what would of been 209FoW, is now W092oF.

SO congrats, now just put
[PHP]echo $captcha;[/PHP]
and you get a random captcha each time. But wait, its in text form, that's not useful, seeing as any bot can copy that code, but what they cant copy is a image. So here is how to make it an image like above.

Our code should look something like this right now
[PHP]<?php
$length = 3;
$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVW XYZ';
$charslength = strlen($chars);
$randomstring = '';
for ($i = 0; $i < $length; $i++) $randomstring .= substr($chars, rand(0, $charslength - 1), 1);
$number = rand(100, 999);
$captcha = $number . $randomstring;
$captcha = str_shuffle($captcha);
?>[/PHP]

Lets add on to it, just enter abit, making some space between both codes.

[PHP]$handle = ImageCreate (90, 24) or die ("Cannot Create image");
$bg_color = ImageColorAllocate ($handle, 255, 0, 0);
$txt_color = ImageColorAllocate ($handle, 0, 0, 0);[/PHP]
Here we are making $handle creation a image that is 90 by 24.
bg_Color is the color of the background in RBG format, knowing that we know
255,0,0 = Red.
txt_color is showing to have 0 color for RBG, making it black.

Now we have our colors set and the handle set lets add the text then create the image.
[PHP]ImageString ($handle, 8, 8, 5, "$captcha", $txt_color);
header ("Content-type: image/png");
ImagePng ($handle);
[/PHP]

ImageString is telling us, we will use handle as our base, with 8 font. The next 2 numbers is the location, which is 8 Y axis, and 5 X axis. That is equivalent to if you go to MS Paint, put a selection just over a image, and see those 2 numbers below change. its x, y.

next we are using a header, to tell the PHP file to act as a PNG file.
Last code is to create the file using the ImagePNG function.

Now save the script, upload, and go to it.
Mine is
which is
Full Code:
[PHP]<?php
$length = 3;
$chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVW XYZ';
$charslength = strlen($chars);
$randomstring = '';
for ($i = 0; $i < $length; $i++) $randomstring .= substr($chars, rand(0, $charslength - 1), 1);
$number = rand(100, 999);
$captcha = $number . $randomstring;
$captcha = str_shuffle($captcha);


$handle = ImageCreate (90, 24) or die ("Cannot Create image");
$bg_color = ImageColorAllocate ($handle, 255, 0, 0);
$txt_color = ImageColorAllocate ($handle, 0, 0, 0);
ImageString ($handle, 8, 8, 5, "$captcha", $txt_color);
header ("Content-type: image/png");
ImagePng ($handle);
?>[/PHP]


Ill make another tutorial teaching how to put it to use.