PHP generated Chinese Verification Code program

<?php
Session_Start ();
# get random characters
function Getrandstr ($length = 4, $mode = 1)
{
$str 1 = ' 123456789 ';
$str 2 = ' abcdefghijklmnopqrstuvwxyz ';
$str 3 = ' abcdefghijklmnpqrstuvwxyz ';
$str 4 = ' _ ';
$str 5 = ' ~!@#$%^&* ()-+=\| {}[];:'",./?';
$str 6 = ' Jihai, don't give the Japanese any chance. Great China Right-back he inherits the glorious tradition of the Chinese nation Dongcunrui Huang Jiguang autumn Little cloud at this moment the soul protrusions Jihai A man he represents the long tradition of Chinese football in this moment he is not a man of the fight he's not alone. Liyi the emperor face the penalty, he faces the eyes and expectations of the Chinese fans all over the world. Cks can live once in the supplementary game of the World Cup caucuses in the additional game to throw out countless penalties the great must know that he can still smile to face the person in front of him? What's he going to be after the second? The Chinese team won the victory of the Japanese team they did not once again to the great Chinese team of the People's Republic of China in front of Zico Right back Kitty today Happy birthday to the People's Republic of China Long live the Japanese team may regret Zico he played too conservatively in the second half, and he lost himself. The courage to face the long history and tradition of Chinese football he didn't hit the goalkeeper like he did in a group game. They should go home. They don't have to go back to Japan because they Will choose collective Caesarean section suicide goodbye ';

Switch ($mode)
{
Case ' 0 ':
$str = $str 1. $str 2. $str 3. $STR 4;
Break

Case ' 1 ':
$str = $str 1;
Break

Case ' 2 ':
$str = $str 2;
Break

Case ' 3 ':
$STR = $str 3;
Break

Case ' 4 ':
$str = $str 2. $str 3;
Break

Case ' 5 ':
$str = $str 1. $str 2;
Break

Case ' 6 ':
$str = $str 1. $str 3;
Break

Case ' 7 ':
$str = $str 1. $str 2. $str 3;
Break

Case ' 8 ':
$str = $str 1. $str 2. $str 3. $str 4. $str 5;
Break
Case ' 9 ':
$str = $str 6;
Break
Default:
$str = $str 1. $str 2. $str 3. $STR 4;
Break
}

$result = "";

if ($mode ==9)
{
$tmp 1 = $length/2;
$tmp = Explode (".", strlen ($STR)/2);
for ($i = 0; $i < $tmp 1; $i + +)
{
$result. = Substr ($str, Mt_rand (0, $tmp [0]) *2-$length, 2);
}
}
Else
{
for ($i = 0; $i < $length; $i + +)
{
$result. = $str [Mt_rand (0, strlen ($str)-1)];
}
}
return $result;
}

##
# Generate CAPTCHA Picture
#
function createauthnumimg ($randStr, $isGB =false)
{
Header ("Content-type:image/png");

$im = Imagecreate (50,20);
$black = Imagecolorallocate ($im, 0,0,0);
$white = Imagecolorallocate ($im, 255,255,255);
$gray = Imagecolorallocate ($im, 0,100,150);
Imagefill ($im, 50,19, $gray);

For ($i =0 $i <80; $i + +) # Add interference pixels
{
$randcolor = Imagecolorallocate ($im, Rand (0,255), Rand (0,255), Rand (0,255));
Imagesetpixel ($im, Rand ()%50, Rand ()%19, $randcolor);
}

if ($isGB!=false) # using Chinese verification Code
{
Require_once ("GB2312toUTF8.inc.php");
$GB 2312toUTF = new Gb2312toutf8 ();
Imagettftext ($im, 5, 5, $white, "SIMSUN.TTC", $GB 2312toutf->gb2utf8 ($RANDSTR));
}
else Imagestring ($im, 8, 8, 3, $RANDSTR, $white); # Draw the validation code into the picture

Imagepng ($im);
Imagedestroy ($im);

$_session[' chknum ' = $randStr; # Save Verification Code
}
$m =$_get["M"];
if (empty ($m)) {
$m = 9;
}
Createauthnumimg (Getrandstr ("6", $m), 1)
?>