PHP Verification Code pattern cannot be displayed

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);
?>

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------Solution--------------------
Comment out the header run the newspaper what error
------Solution--------------------
$_session[' pass_phrase ' = SHA ($pass _phrase);
Change to:
$_session[' pass_phrase '] = SHA1 ($pass _phrase);
It?

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