PHP Verification Code pattern cannot be displayed

The following captcha.php file runtime returns 500 error, does not display the CAPTCHA pattern
Note the phrase "$_session[' pass_phrase '" = SHA ($pass _phrase); "After, will not error and can display the pattern, what reason?"
How do I change the session to save information?

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 < $i + +) {
Imagesetpixel ($img, rand ()% Captcha_width, rand ()% captcha_height, $graphic _color);
}
Draw the Pass-phrase string
Imagettftext ($img, 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);
?>

Reply to discussion (solution)

Take the header (' Content-type:image/png ') and get to the front.

Comment out the header run the newspaper what error

Take the header (' Content-type:image/png '), get to the front or 500, and Firefox tip:
The image "http://localhost/captcha.php" cannot be displayed because of its own problem

I didn't get the tip when I got the front.
In addition, when you get to the front, if you note the phrase "$_session[' pass_phrase '" = SHA ($pass _phrase); ", you can still display the

Note the header or report 500:
GET captcha.php
Internal Server Error
localhost
0 B
127.0.0.1:80

Response header Information Raw header information
Cache-control No-store, No-cache, Must-revalidate, post-check=0, pre-check=0
Connection Close
Content-length 0
Content-type text/html
Date Sun, Geneva 12:16:16 GMT
Expires Thu, Nov 1981 08:52:00 GMT
Pragma No-cache
Server apache/2.2.25 (WIN32) php/5.3.27
X-powered-by php/5.3.27
Request header information Raw header information
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-encoding gzip, deflate
Accept-language zh-cn,zh;q=0.8,en-us;q=0.5,en;q=0.3
Cache-control max-age=0
Connection keep-alive
Cookie Phpsessid=9e4i75adp4ahnpba6ieq1pinn0
Host localhost
User-agent mozilla/5.0 (Windows NT 6.3; WOW64; rv:24.0) gecko/20100101 firefox/24.0

$_session[' pass_phrase ' = SHA ($pass _phrase);
Change to:
$_session[' pass_phrase '] = SHA1 ($pass _phrase);
It?

, SHA () is the function of MySQL, PHP is SHA1, thank you, give points

