PHP验证码图案没法显示

PHP验证码图案无法显示
下面captcha.php文件运行时回报500错误, 不显示验证码图案
注掉这句"$_SESSION['pass_phrase'] = SHA($pass_phrase); "后,就不报错并且可以显示图案了,什么原因呢?
需要session保存信息要如何修改?

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
$_SESSION['pass_phrase'] = SHA($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);
?>

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------解决方案--------------------
将header注释掉运行看报什么错误
------解决方案--------------------
$_SESSION['pass_phrase'] = SHA($pass_phrase);
改成：
$_SESSION['pass_phrase'] = SHA1($pass_phrase);
呢？