PHP highly customizable security verification code code_PHP tutorial

public function captcha(){
$font_dir = $_SERVER ["DOCUMENT_ROOT"] . "your_ttf_file.ttf"; // Font library
$img_w = ​​58; // Set image width
$img_h = 20; // Set image height
$font_size = 11; // Font size
$angle_l = -10; // Left declination angle
$angle_r = 10; // Right declination angle
$code_str = "ABCDEFGHJKLMNPQRSTUVWXYZ36";
$word_len = 4; / / Number of verification code digits
$padding = 5; // Space between each two words
$margin = 2; // Left margin
$base_line = 15; // Text baseline position
$base_line_offset = 2; // Baseline offset
$pixel_num = 3; // Base number of noise points
$pixel_color= 8; // There are only $pixel_color colors of noise points. The total number of noise points is $pixel_num*$pixel_color
$noise_font_size = 1; // Noise font size
$session_key= "my.xoyo_captcha"; // Custom session key name

header("Cache-Control : no-cache, must-revalidate");
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Pragma: no-cache");
header("Cache-control: private");
header('Content-Type: image/png');

session_start();
$word = "";
$ code_str_len = strlen($code_str) - 1;
for ($i = 0; $i < $word_len; $i++) {
$word.= $code_str[rand(0, $code_str_len)];
}
$_SESSION [$session_key] = strtolower($word);
$image = imagecreatetruecolor($img_w, $img_h);
imagefilledrectangle($image, 0, 0, $img_w - 1, $img_h - 1, imagecolorallocate($image, mt_rand(235, 255), mt_rand(235, 255), mt_rand(235, 255)));

//Draw noise
for ($i = 0; $i < $pixel_color; $i++){
$noise_color = imagecolorallocate( $image, mt_rand(150,225), mt_rand(150,225), mt_rand(150,225) );
for($ j = 0; $j < $pixel_num; $j++) {
imagestring( $image, $noise_font_size, mt_rand(-10, $img_w), mt_rand(-10, $img_h), $code_str[mt_rand(0 , $code_str_len)], $noise_color );
}
}

//Draw text
for ($i = 0; $i < $word_len; ++$i) {
$color = imagecolorallocate($image, mt_rand(0, 100), mt_rand(20, 120), mt_rand(50, 150));
imagettftext($image, $font_size, mt_rand($angle_l, $angle_r), $margin, mt_rand($base_line-$base_line_offset, $base_line+$base_line_offset), $color, $font, mb_substr($word, $i, 1, 'utf-8'));
$margin += (imagefontwidth($font_size) + $padding);
}

imagepng($image);
imagedestroy($image);
exit;
}