(Advanced article) PHP generated CAPTCHA code program

The following text:

Create a new PHP file captcha_code_file.php

First open Sessionsession_start ();//define the foreground display verification code length & Width $image_width = +; $image _height = +; $characters _on_image = 6; $font = './monofont.ttf ';//the characters that can is used in the CAPTCHA code.//avoid confusing characters (l 1 and I for example ) $possible _letters = ' 23456789bcdfghjkmnpqrstvwxyz '; $random _dots = ten; $random _lines = $captcha _text_color= " 0x142864 "; $captcha _noice_color =" 0x142864 ";//define the string to generate the verification code $code = '; $i = 0;while ($i < $characters _on_image) {$code. = substr ($possible _letters, Mt_rand (0, strlen ($possible _letters)-1), 1); $i + +;} $font _size = $image _height * 0.75; $image = @imagecreate ($image _width, $image _height);/* Setting the background, text and n Oise colours here */$background _color = imagecolorallocate ($image, 255, 255, 255); $arr _text_color = Hexrgb ($captcha _text _color); $text _color = Imagecolorallocate ($image, $arr _text_color[' Red '], $arr _text_color[' green ', $arr _text_color [' Blue ']); $arr _noice_color = Hexrgb ($captcha _noice_color); $image _noise_color = ImagecoLorallocate ($image, $arr _noice_color[' red '), $arr _noice_color[' green '], $arr _noice_color[' Blue ');/* Generating th e dots randomly in background */for ($i =0; $i < $random _dots; $i + +) {imagefilledellipse ($image, Mt_rand (0, $image _width) , Mt_rand (0, $image _height), 2, 3, $image _noise_color);} /* generating lines randomly in background of image */for ($i =0; $i < $random _lines; $i + +) {imageline ($image, Mt_rand (0, $image _width), Mt_rand (0, $image _height), Mt_rand (0, $image _width), Mt_rand (0, $image _height), $image _noise_color);} /* Create a text box and add 6 letters code in IT */$textbox = Imagettfbbox ($font _size, 0, $font, $code); $x = ($image _width-$textbox [4])/2; $y = ($image _height-$textbox [5])/2;imagettftext ($image, $font _size, 0, $x, $y, $text _color, $font, $code);/* Show captcha image in the page HTML page */header (' content-type:image/jpeg ');//Defining the IM Age type to being shown in browser widowimagejpeg ($image);//showing the Imageimagedestroy ($image);//destroying the image instance//set SESSION, do verify $_session[' 6_letters_code '] = $code; function Hexrgb ($hexstr) {$int = Hexdec ($HEXSTR);               Return Array ("Red" = 0xFF & ($int >> 0x10), "green" + 0xFF & ($int >> 0x8), "Blue" = 0xFF & $int);}

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