一个简单的php图形验证码生成程序

<?php
/**
 *
 * @file imgvcode.php
 * @create date 2007-09-25
 * @copyright (c) 2005 - 2007 eifr.com
 * @license http://www.hzhuti.com/nokia/n97/
 * eifr is free software
 */
session_start();
// main
$vcodes = '';
//generate Number 4
srand((double)microtime()*1000000);
for($i=0;$i<4;$i++){
    $vcodes.=rand(1,9);
}
$_SESSION['eifr_checkvcode'] = $vcodes;
if(function_exists('imagecreate')){
    //generate picture validation code
    Header("Content-type: image/PNG");
    $img = imagecreate(44,18);
    $bg = ImageColorAllocate($img, 245,245,245);
    imagefill($img,0,0,$bg); //background
   
    //generate Number 4
    for($i=0;$i<4;$i++){
        $font = ImageColorAllocate($img, rand(100,255),rand(0,100),rand(100,255));
        $vnum = substr($vcodes, $i, 1);
        imagestring($img, 5, 2+$i*10, 1, $vnum, $font);
    }
    //add interference
    for($i=0;$i<100;$i++)
    {
        $randcolor = ImageColorallocate($img,rand(0,255),rand(0,255),rand(0,255));
        imagesetpixel($img, rand()%70 , rand()%30 , $randcolor);
    }
    ImagePNG($img);
    ImageDestroy($img);
}
?>