GD library in PHP5 generates graphic verification code (with Chinese characters)_PHP tutorial

//1.qi Enable the gd library. The GD library provides a series of APIs for processing images. You can use the GD library to process images or generate images. .
// On websites, the GD library is usually used to generate thumbnails or to add watermarks to images or to generate reports on website data.
session_start();
// Convert the GBK encoded string into a UTF-8 string. The first parameter is written GBK because the encoding of this php file stored in the host is GBK encoding
// UTF-8 encoding is generally supported by browsers and has strong versatility. Let’s convert it to UTF-8 here
$str = iconv("GBK", "utf-8", "Everyone is open to green waters, green mountains, scenic spots and historic sites Your heart will be filled with joy and happiness will always be with you");
if(!is_string($str) || !mb_check_encoding($str,"utf-8"))
{
exit("No The string may not be utf-8");
}
$zhongwenku_size;
// Get the length of the string in UTF-8 encoding
$zhongwenku_size = mb_strlen($str,"UTF- 8");
// Import the above characters into the array
$zhongwenku = array();
for( $i=0; $i<$zhongwenku_size; $i++)
{
$zhongwenku[$i] = mb_substr($str, $i,1,"UTF-8");
}
$result = "";
// The four characters to be written on the picture characters
for($i=0; $i<4; $i++)
{
switch (rand(0, 1))
{
case 0:
$ result.=$zhongwenku[rand(0, $zhongwenku_size-1)];
break;
case 1:
$result.=dechex(rand(0,15));
break;
}

}
$_SESSION["check"] = $result;

// Create a true color image with a width of 100 and a height of 30
$img = imagecreatetruecolor (100, 30);
// Assign background color
$bg = imagecolorallocate($img, 0, 0, 0);
// Assign text color
$te = imagecolorallocate($img , 255,255,255);
//Write string on the image
//imagestring($img, rand(3,8), rand(1,70), rand(1,10), $result, $ te);
// Special fonts can be written on the image according to the loaded font
imagettftext($img, 13, rand(2, 9), 20,20, $te, "MSYH.TTF", $result);
$_SESSION["check"] = $result;
for($i=0; $i<3; $i++)
{
// $t = imagecolorallocate( $img, rand(0, 255),rand(0, 255),rand(0, 255));
// Draw line
imageline($img, 0, rand(0, 20), rand (70,100), rand(0, 20), $te);
}
$t = imagecolorallocate($img, rand(0, 255),rand(0, 255),rand(0, 255) );
// Add noise to the image
for($i=0; $i<200; $i++)
{
imagesetpixel($img, rand(1, 100), rand( 1, 30), $t);
}
// Send http header information to specify that the jpeg in the image is sent this time
header("Content-type: image/jpeg");
// Output jpeg images to the browser
imagejpeg($img);
?>

session_start();
if(@$_POST['check'])
{
if($_POST["check"] == $_SESSION["check"])
{
echo" Congratulations! The verification code is entered correctly! ";
}else{
echo" Sorry, the verification code was entered incorrectly";
}
}
?>

< ;input type="submit" value="submit">