我正在尝试使用PHP教程书，而且我一直遇到一个问题，我很难解决这个问题。
这些是我得到的错误代码：
Warning: imagefilledrectangle() expects parameter 1 to be resource, null given in /home/london/public_html/iweb21.com/ch11/captcha.php on line 19
Warning: imageline() expects parameter 1 to be resource, null given in /home/london/public_html/iweb21.com/ch11/captcha.php on line 22
Warning: imagesetpixel() expects parameter 1 to be resource, null given in /home/london/public_html/iweb21.com/ch11/captcha.php on line 27
代码：
<?php
session_start();
// Set some important CAPTCHA constants
define('CAPTCHA_NUMCHARS', 6); // number of characters in pass-phrase
define('CAPTCHA_WIDTH', 100); // width of image
define('CAPTCHA_HEIGHT', 25); // height of image
// Generate the random pass-phrase
$pass_phrase = "";
for ($i = 0; $i < CAPTCHA_NUMCHARS; $i++) {
$pass_phrase .= chr(rand(97, 122));
}
// Store the encrypted pass-phrase in a session variable
$_SESSION['pass_phrase'] = sha1($pass_phrase);
// Create the image
$img = imagecreatetruecolor(CAPTCHA_WIDTH, CAPTCHA_HEIGHT);
// Set a white background with black text and gray graphics
$bg_color = imagecolorallocate($img, 255, 255, 255); // white
$text_color = imagecolorallocate($img, 0, 0, 0); // black
$graphic_color = imagecolorallocate($img, 64, 64, 64); // dark gray
// Fill the background
imagefilledrectangle($img, 0, 0, CAPTCHA_WIDTH, CAPTCHA_HEIGHT, $bg_color);
// Draw some random lines
for ($i = 0; $i < 5; $i++) {
imageline($img, 0, rand() % CAPTCHA_HEIGHT, CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color);
}
// Sprinkle in some random dots
for ($i = 0; $i < 50; $i++) {
imagesetpixel($img, rand() % CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color);
}
// Draw the pass-phrase string
imagettftext($img, 18, 0, 5, CAPTCHA_HEIGHT - 5, $text_color, 'Courier New Bold.ttf', $pass_phrase);
// Output the image as a PNG using a header
header("Content-type: image/png");
imagepng($img);
// Clean up
imagedestroy($img);
?>